You can shrink the size of your MP4 files by using AppGeeker video converter, it has options to manually set the file size to fit your need.
However, please keep in mind that the smaller the file, the greater the loss of quality. I dont know what is size of your original mp4 files, you may consider split the file into some small parts to email (AppGeeker also handle that stuff).
As for uploading MP4 files into YouTube, you can directly do that on your computer with interrupt. But it would be better to convert them to a .flv file extension (the mostly used format for distributing videos online) that YouTube accepts well.
See: How to Convert FLV to MP4 (or MP4 to FLV ) for Mac/ Windows
Hope it is useful.