I take it you didn't search this forum first before posting?
Can you record in MP4? sure. Should you? no, definitely not, and why you get a warning if you even try. do some research of why that is a terrible idea
general recommendation is to record to the default MKV container, then ask/let OBS remux to MP4 once recording completed.
Then, you should do some research on basic video format encoding standards, and containers (ie MKV and MP4 could have same encoded video inside their respective container (I think I'm saying that right). Bottom line, different video editors have different requirements / support different 'standards'. Asking for a MP4 filename is NOT detailed enough, you need next level down (encoding details) to ensure compatibility. Even within certain encoding standards, there are options and you have to ensure a match there as well
So, basically not as simple a question, really. And as for what settings to use? depends on what you are trying to accomplish, and what compromises you prefer (ie higher quality video at cost of disk space and CPU power to encode/decode... )
