If this is an mp4 file from say... a YouTube download?
YouTubeDownloader the free version which you can get at cnet downloads, filehippo and elsewhere does a very nice job of converting them.
Select "optimal" for quality level
The only rub with the free version of the program is that it does not do batch conversions, meaning you cannot browse for files and Shift+Click multiple files
but if after you start a conversion you can immediatly (while the first one is converting) select the conversion tab browse for another file and start that one converting as well, etc....
I can keep three running at any given time (I have a quad core computer) if I am quick with my mouse and any additional files are simply queued until one of the others that are in-process has completed.
I'm not sure if the $19.95 paid version changes this or not.
But YouTube downloader works well enough that I've stopped looking for something better and if I can get a definate answer on batch conversions I'll probably pay them their $20.