Your 4K YouTube download won't open in QuickTime. Here's why.
YouTube serves nothing above 1080p in H.264. Above that it is VP9 and AV1, which QuickTime cannot play. Three ways out, and what each costs.
You downloaded a video in 4K. QuickTime opens, shows a black window, and says the file is not compatible. The file is fine. QuickTime is telling the truth.
The one fact behind all of it
YouTube does not serve H.264 above 1080p.
H.264 is the codec everything on a Mac plays, because it is decoded by dedicated hardware in the chip. Every camera writes it, every editor imports it, QuickTime opens it instantly and your battery barely notices.
Above 1080p, YouTube switches to VP9 and, increasingly, AV1. Both are newer, both compress better, and QuickTime Player plays neither. So the moment you ask for 1440p or 2160p, you are no longer asking for a bigger file — you are asking for a different codec, and that is the whole problem.
Nothing about your download went wrong. The resolution and the codec are the same choice, and nobody tells you.
Check what you actually have
ffprobe -v error -select_streams v:0 \
-show_entries stream=codec_name,width,height \
-of default=noprint_wrappers=1 video.mp4
codec_name=h264 opens anywhere. vp9 or av01 is the file in front of you.
Three ways out
1. Use a player that speaks VP9 — free, instant, no downside
IINA is the Mac video player most people end up on, and it plays everything without converting anything. VLC does the same job with a worse interface.
This is the right answer for anything you intend to watch. The file is already perfect; only QuickTime is the problem, and QuickTime is not the only player.
The one thing to know: VP9 and AV1 have little to no hardware decoding on older Apple Silicon, so 4K playback runs on the CPU. On a laptop, on battery, you will hear the difference.
2. Re-encode to H.264 — works everywhere, costs quality and time
If the file has to go into Final Cut, or onto something that only speaks H.264:
ffmpeg -i input.webm -c:v libx264 -crf 18 -preset slow \
-c:a aac -b:a 192k output.mp4
-crf 18 is visually near-lossless; lower is bigger, higher is smaller.
Be clear about what this does. You are decoding a compressed video and compressing it again with a different codec, which always loses something — generational loss, and it is irreversible. On a 4K file it also takes a while. For an edit that has to happen, this is the price. For something you are going to watch once, it is a waste of an hour.
Faster, with quality closer to the source, using the encoder in the chip:
ffmpeg -i input.webm -c:v h264_videotoolbox -b:v 20M \
-c:a aac -b:a 192k output.mp4
VideoToolbox is dramatically faster and slightly worse at the same bitrate.
Real-time work: VideoToolbox. Archiving: libx264.
3. Take 1080p H.264 instead — the one most people should pick
yt-dlp -f "bestvideo[vcodec^=avc1]+bestaudio[ext=m4a]/best[ext=mp4]" "URL"
vcodec^=avc1 is H.264. This asks for the best quality that is still H.264,
which in practice means 1080p, and hands you a file that opens in QuickTime,
imports into any editor, and plays on any phone or TV you point it at.
Before dismissing it: YouTube gives 1080p a healthier bitrate than it used to, and on any screen smaller than a large monitor, at normal viewing distance, the difference between that and 2160p VP9 is far smaller than the difference between a file that works everywhere and a file that needs a special player.
What to choose
| You want to | Take |
|---|---|
| Watch it | 4K as it comes, in IINA |
| Edit it | 1080p H.264, or re-encode if you truly need the resolution |
| Send it to someone | 1080p H.264, every time |
| Archive it | 4K as it comes. Never re-encode for storage. |
The last line matters most. Re-encoding to make a file convenient today throws away quality you cannot get back. Keep the original, convert a copy.
Why a downloader should default to H.264
This is why the app I build picks H.264 and AAC unless you ask for something else: the file that opens on the first double-click is worth more than the file with the bigger number on it, for almost everyone, almost always.
But it is a default, not a rule. When you actually need 2160p, you need it, and the honest thing is to hand it over along with the reason QuickTime is about to refuse it.