FLV is the format that gets streamed to servers that serve RTMP streams. It's easiest to output files as FLV from OBS because that is the streaming file format.
Native MP4 output is coming in a future update, but for now you can use FFmpeg output (Settings > Output > Advanced > Recording > Type: FFmpeg) to output as MP4 for now. Note that there is currently an issue with FFmpeg output that causes the same file to get overwritten rather than renamed each time you record over it.