i know lots.
and i know mp4 is horsestuff, be it video OR audio.
If AAC (which is what the audio codec is called, if you did indeed know jack stuff about it) is stuff then why don't you do a listening test for me at 256kb/s which is the bitrate of audio that apple releases in its music store and see if you can tell any damn difference
Video codecs are an entirely different story. Video at high resolutions inherently needs a much higher bitrate than what youtube provides to look the same to the original. Audio is generally identical sounding to the original at 128kb/s, video (depending on the resolution) can range from 4000-8000kb/s (or higher if it's 4k resolution) because you just need that much more information to make a really nice looking mp4.