how do I convert .mov to .mp4?

Use any quality setting but not Best (ProRes).

The so called "Best (ProRes)" setting should never be used normally. It is only for use if you want to keep a copy of the project for re-editing.

If you go to File > Share > File, anything less than Best (ProRes) will save as an .mp4.

If you need the ProRes, you will need to use another piece of software like HandBrake or VLC.