how to convert .mov to an mp4 file

Why would you want to do that?

Anyways, the following command does that pixel-wise losslessly and very fast because it does not do any re-coding but just changes the wrapper to .mp4. Ask more if you want to preserve metadata like dates, locations etc.

ffmpeg -i input.mov -c copy output.mp4

Another option is to re-encode .mov to .mp4 with apps like Handbrake or Shutter Encoder but that is slower and lossy.

A quick and dirty option is to just change the suffix from .mov to .mp4 but that does not really change it to .mov. But it might work if some site insists using only .mp4 and is happy with that.