My mp4 won't play the video only the sound? how do I get it to play video?

It depends on what codecs the .mp4 has and what macOS version you use. For example:

QuickTime Player refuses to play some H.265/HEVC flavors. macOS 11 Big Sur is most forgiving but has the following issues with "Codec ID" and "Chroma subsampling" options:

- H.265/HEVC with Codec ID hvc1 plays OK.

H.265/HEVC with Codec ID hev1: "This file contains media which isn't compatible with QuickTime Player" plays audio only (maybe this is the issue with your .mp4??). Fix: add '-tag:v hvc1' without re-encoding:

ffmpeg -i input.mp4 -vcodec copy -acodec copy -tag:v hvc1 output.mp4

- Chroma subsampling 4:2:0 plays OK.

Chroma subsampling 4:2:2: "This file contains media which isn't compatible with QuickTime Player". Opens anyway in Big Sur but fails in Mojave.

- VLC and IINA have no issues with either.

Then there are many old obsolete codecs that Mojave offered to convert because they no longer work in Catalina or later. The fix them, boot into Mojave or earlier and use MPEG Streamclip or QT Player 7 Pro to convert them to H.264 or H.265/HEVC .mp4 or use VLC, Handbrake, ffmpeg etc to convert them.

Some MP4 files have audio formats that are not supported by QuickTime Player.

Would you mind using a different media player for this? Try VLC.

Do you wish to analyze this further? Then get and share file details from a media inspector app, e.g. Invisor Lite.