converting .mov to .mp4

I'm on an iPhone 6, iOS 11.2.6 shooting 1 minute videos. My partner shot the same thing on his iPhone and it came out as a .mp4. I shot a 1 minute video and it came out a .mov. I need to convert or figure out how to reshoot and have it be a .mp4. I've been to all the settings, reducing the HD footprint, but there is no taking it OFF HD, which has been recommended to make the file smaller. I've been all over the internet and I don't know where else to look. Thnx