MP4 files already use a highly compressive codec like H.264, thus further compressing the video will get you more loss of visual quality.
Do you know, what resolution and length and frame rate the video has? If so post that info. If not, go to QuickTime Player and open the file there and press CMD+I to get the Movie Inspector.
In there you find
.
Post that info here or make a
screenshot and
attach it to your next post.
This video might also have some insights for you:
PS: Is the file by any chance a 720p video with a length of approx. 43 minutes?