I'm a newbie, so just trying to help get you started... Ignoring OBS, I'd start with basic troubleshooting
- does the video MP4 file play fine on the PC outside of OBS (Win Media player, or other?)
- Have you tried other videos (from other sources, in case compatibility issue between settings used with current encoder vs OBS's settings/expectations...
- Have you tried using the VLC player instead to play the video?
For reference, I use Win10 (various releases in last 5 months), OBS 25.0.8 and I've had no problem with MP4 videos from multiple sources (iOS devices, Android, Premiere, etc). Though sometimes, I've seen OBS struggle with first playback of a video. So, I'm in the habit of checking (pre-playing part of) every pre-recorded video .. and having video set to not close when done playing, for each livestream. I now have a more powerful computer, without some over the top security s/w installed, and still in the habit... not sure if I need to or not, but I like to pre-run a livestream in advance regardless, so not really an imposition from my perspective