Can someone explain what the MP4 format actually is

Mac
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I keep seeing videos labeled as MP4, but I’m not clear on what the MP4 format actually means or how it’s different from other formats like AVI or MKV. I’m trying to choose the best format for saving and sharing videos without losing too much quality or running into playback issues. Can someone break down what MP4 is, how it works, and when I should use it?

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MP4 shows up everywhere once you start paying attention to your files. I noticed it first when I pulled videos off my phone and again when I started hoarding random clips from group chats and sites. At some point, almost every folder on my drive had at least one MP4 sitting in it.

MP4 is not one single “type” of video. It is more like a box format that holds several streams together in one file. Inside one MP4 file you often have:

  • Video stream (H.264, HEVC, or something else)
  • Audio stream (AAC, MP3, etc.)
  • Optional subtitle tracks
  • Optional extras like chapters, cover art, or thumbnails

So when you see .mp4, you only know the container, not what codecs live inside. That explains why two MP4 files can behave very differently on slower hardware. Same extension, completely different load on your CPU.

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Below is what has worked for me on Windows and macOS. None of this is sponsored, I just broke things enough times to figure out what causes fewer headaches.

macOS: my experience with MP4 players

On Mac things are slightly different. Apple already gives you QuickTime Player, which handles MP4 quite well as long as you stay inside “normal” use.

Elmedia Player

I installed Elmedia when QuickTime refused to open a couple MKV and weird MP4 files someone sent me.

What I liked:

  • It opens MP4, MKV, AVI, and a pile of other formats
  • No external codec packs or extra tools needed
  • Drag and drop subtitles, or let it fetch subtitles from online databases
  • You can adjust playback speed with simple shortcuts
  • Brightness, contrast, saturation, and similar video tweaks are right in the UI
  • Playlists feel more comfortable than QuickTime for back to back episodes

The part that surprised me was the wireless streaming. I used it to send MP4 files from my Mac to:

  • Apple TV
  • Chromecast
  • A couple “smart” TVs on the same network

I had less stutter using Elmedia to stream local MP4s than using some of the TV’s built in apps.

QuickTime Player on macOS

QuickTime Player ships with macOS, and for simple MP4 files it does fine.

Where it works:

  • Phone recordings
  • Screen recordings
  • Standard MP4 downloads with common codecs

I use it when:

  • I want to trim the start or end of a clip; QuickTime’s trimming feels quick
  • I only need one audio track and no fancy subtitles
  • I am showing someone a clip and do not want to install extra software

Where it hits a wall:

  • No native support for MKV or some AVI files
  • Certain MP4 files that use odd codecs inside refuse to play or need conversion
  • Subtitle handling is weak compared to other players

So on Mac my pattern looks like this:

  • QuickTime Player for basic MP4 clips and screen recordings
  • Elmedia for everything else, especially when I need support for multiple formats or streaming

Windows: what I use for MP4

On Windows I stopped relying on the default “Movies & TV” app pretty fast. It plays MP4s, but once you get outside simple files, it starts to feel limited.

Here is what I use instead, depending on the situation.

VLC Media Player

If I do not want to think about anything, I open stuff in VLC.

What it handles well for MP4:

  • MP4, MKV, AVI, pretty much every codec I have thrown at it
  • Broken or half-downloaded files sometimes still play
  • Subtitle files like SRT and ASS load fine
  • Multiple audio tracks inside one MP4

What I like in day to day use:

  • No hunting for codec packs
  • Keyboard shortcuts for everything
  • Can change playback speed, deinterlace, crop, etc.
  • Decent on older hardware if you keep the settings modest

Where it annoys me:

  • Interface feels stuck in time
  • Some hardware acceleration options are buried in menus
  • Color sometimes looks off until you tweak it

Still, if someone asks me “how do I open this weird MP4,” VLC is usually the first suggestion.

PotPlayer

When I used a mid range Windows laptop with an integrated GPU, PotPlayer felt lighter than VLC for certain high bitrate MP4 files.

Why I ended up using it often:

  • Faster seek times on large MP4 files
  • Smooth playback on 1080p and some 4K content
  • Lots of video filter options if you care about picture tuning
  • Per file settings remembered surprisingly well

Stuff to be aware of:

  • Tons of settings, easy to get lost if you over tweak
  • The installer used to have bundled extras, so I always read every screen
  • Interface takes a bit to configure how you like it

If you watch a lot of anime, sports, or high bitrate rips, PotPlayer tends to handle MP4s without stuttering as long as your hardware is not ancient.

How I think about MP4 now

When I organize media or move videos between devices, I treat MP4 as a container that keeps:

  • Video
  • Audio
  • Subtitles
  • Extras

all in one place. That is why it shows up on phones, consoles, TVs, browsers, and random apps.

If you keep running into MP4 playback issues, check:

  • The actual video codec inside the MP4 file
  • Hardware acceleration support on your system
  • Whether your player supports multiple tracks and subtitles properly

Once you adjust those three points and pick a decent player for your OS, MP4 files stop being a mystery and start feeling like regular files you move around and open without thinking about it.

1 Like
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MP4 is a “container” format. Think of it as a file structure that holds different streams together in one .mp4 file.

Inside a single MP4 you can have:

  • Video codec, usually H.264 or H.265/HEVC
  • Audio codec, often AAC or sometimes MP3
  • Optional subtitles
  • Optional metadata like chapters or cover art

So when you see .mp4 you know the container, not the quality or the codec. A 4K H.265 MP4 and a 480p H.264 MP4 behave very differently for your CPU and for old devices.

How it compares to AVI and MKV

AVI:

  • Older format
  • Limited support for modern streaming features
  • Works fine for local playback, not great for web or mobile
  • Larger files for same quality in many cases

MKV:

  • Very flexible container
  • Great for multiple audio tracks, advanced subtitles, extras
  • Widely used for “rips” and archives
  • Less support on TVs, consoles, some phones

MP4:

  • Supported almost everywhere. Phones, browsers, TVs, consoles, social media
  • Well suited for streaming and sharing
  • Often required by sites and apps that accept video uploads
  • A bit less flexible than MKV for complex setups, but more compatible

If your goal is “saving and sharing videos with other people”, MP4 is usually the safest bet:

  • Use H.264 video plus AAC audio inside the MP4
  • Resolution 1080p for normal stuff, 720p if storage or bandwidth is tight
  • Bitrate around 5 to 8 Mbps for 1080p is a decent balance

If you want to archive media with multiple audio tracks, lossless audio, or fancy subtitles, MKV is better. For normal personal clips and sharing, I would not touch AVI anymore.

On players, I agree with a lot of what @mikeappsreviewer said, but I lean more on “use one good player and stop tweaking”. On macOS, Elmedia Player is an easy choice. It plays MP4, MKV, AVI and more, handles subtitles, and streams to TVs and Apple TV. For most people it replaces both QuickTime and extra tools.

Practical rule of thumb:

  • For sharing and compatibility: MP4 (H.264 + AAC)
  • For archiving complex files: MKV
  • Ignore AVI unless some old camera forces you to use it

If a specific MP4 does not play on a device, the usual fix is to re-encode the video to H.264 in an MP4 container, not to switch container formats.

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Think of MP4 as a “shipping container” and AVI/MKV as different kinds of containers, not as the video itself.

What MP4 actually is

  • MP4 is a container format (officially ISO/IEC 14496-14, based on the MPEG-4 Part 12 base media format).
  • Inside that container you can pack:
    • Video stream (commonly H.264 / AVC, H.265 / HEVC, sometimes older stuff)
    • Audio stream (AAC, AC-3, MP3, etc.)
    • Optional subtitles (timed text, sometimes others via extensions)
    • Metadata (chapters, title, cover art, rotation info from phones, etc.)

So the .mp4 extension only tells you “this is an MP4 box,” not what is inside the box. Two MP4 files might both be 1080p, but one uses a super heavy codec and your old laptop chokes, the other is easy to play.

Where I partly disagree with the others
@​mikeappsreviewer and @​sognonotturno both framed MP4 as “use it for sharing, MKV for archiving,” which is mostly true, but I’d add:

  • MKV vs MP4 is less dramatic than people make it sound. For a lot of “normal” stuff with only one audio track and maybe basic subtitles, there is basically no practical difference once it plays.
  • The codec choice matters more than the container. A badly encoded H.264 in MKV will look worse than a well encoded H.264 in MP4 at the same size. People blame the container when the real issue is bitrate, encoder settings, or a trash source.

How MP4 compares to AVI and MKV in practice

AVI

  • Ancient (early 90s Microsoft format).
  • Very weak for modern use: poor support for variable frame rate, subtitles, chapters, etc.
  • Often bigger files for the same quality because people pair it with older codecs like DivX/Xvid.
  • Only use it if an old camera or software forces you. For anything new, it is basically a fossil.

MKV

  • Super flexible container.
  • Great for:
    • Multiple audio tracks (languages, commentaries)
    • Advanced subtitles (ASS/SSA, styling, karaoke, etc.)
    • Blu-ray style extras in one file
  • Downsides:
    • Inconsistent support on TVs, consoles, and some phones.
    • Some web platforms still refuse MKV uploads, or transcode them badly.

MP4

  • Designed for streaming and modern distribution.
  • Extremely well supported: browsers, iOS, Android, TVs, consoles, social media.
  • Slightly less flexible than MKV for crazy setups (tons of audio tracks, exotic subtitles), but more than enough for 99% of “I just want to watch this and send it to people” use cases.
  • Plays nicer with hardware acceleration on a lot of devices, especially with H.264 video + AAC audio.

What to actually use, based on what you are doing

  1. Sharing with others / social media / sending around

    • Use MP4.
    • Codec combo: H.264 video + AAC audio inside the MP4 container.
    • Reasons:
      • Uploads without complaint to almost everything.
      • Plays on random phones, laptops, TVs without users having to install anything.
    • This is where I strongly agree with both @​mikeappsreviewer and @​sognonotturno: MP4 is the path of least resistance.

  2. Personal library for general watching on multiple devices

    • Still lean MP4 unless you know you need fancy stuff.
    • Again, H.264 + AAC is the safe combo.
    • If you’re low on disk space and your devices support HEVC (H.265), you can do HEVC-in-MP4 for smaller files at similar quality, but older devices may choke.

  3. Archiving with lots of extras (multi-audio, complex subs)

    • MKV is usually nicer: handles weird subtitle formats and audio combos with less hassle.
    • But if you’re not doing fansub-style subs or 3+ language tracks, MP4 is often fine and more portable.

  4. Avoiding pain when something will not play

    • The instinct “this MP4 doesn’t play, I’ll change it to MKV” is kinda backwards.
    • Container switches alone don’t fix compatibility. The fix is usually:
      • Re-encode the video to H.264
      • Re-encode the audio to AAC
      • Then put those into MP4
    • That’s why some “convert to MP4” tools appear to magically fix everything: they’re actually changing codecs, not just the container.

What about players and Elmedia Player

Since you also seem to care about actually watching these files, not just the theory:

  • On macOS:

    • QuickTime is fine for “normal” MP4s (phone recordings, screen captures).
    • The moment you hit an MP4 that “should” work but doesn’t, or you get MKVs/AVIs, I would jump to Elmedia Player.
      • It plays MP4, MKV, AVI and a ton of others.
      • Handles subtitles and multiple audio tracks much more comfortably than QuickTime.
      • Streaming local MP4s to TVs / Apple TV / Chromecast from Elmedia Player is usually smoother than relying on the TV’s built-in app, which is often garbage.
    • This is where I slightly push back on the “one player and stop tweaking” idea: for macOS specifically, Elmedia Player is good enough that I’d basically treat it as the main player and only fall back to QuickTime for quick trims.

  • On Windows: VLC, PotPlayer, etc., are already covered in detail above by others, so I won’t rehash. Just keep in mind: if a player complains about an MP4, check the codec first.

TL;DR version for deciding your format

  • You want max compatibility, easy sharing, minimal headaches:
    • Use MP4 container
    • With H.264 video + AAC audio
  • You want to store multi-audio, fancy subs, and extras for long-term archiving:
    • Use MKV
  • Ignore AVI unless legacy hardware has you trapped.

If you set your editor/encoder to “MP4 (H.264 + AAC)” and use something like Elmedia Player on macOS or VLC on Windows, you’ll probably stop thinking about formats entirely and just… watch stuff.