.mov is quicktime's native format, therefore it will, by default, always save as a .mov file. If you want to save it as a .mp4 file, just choose "Export" instead of "save".

Apple's QuickTime is a 30-year-old technology but is still supported by Apple. Here's how to use the QuickTime Player included with macOS. Before the internet, and long before streaming, there was ...

^-- see above... i absolutely abhor quicktime, and i can't stand installing it, but i have a few funny clips, and some shorts and movie trailers that i can ...