Quicktime 10 Won't Allow Me To Trim An MP4
I am currently trying to trim an MP4 in Quicktime 10.4 for OS X Yosemite, but the option is greyed-out in the edit menu.
MP4 is a container format that can be created by a variety of methods. Do you know what the origins of the clip are?
Assuming that the clip opens and plays as expected in QuickTime, try the keyboard shortcut, Cmd-T and see whether that brings up the trimming bar.
QuickTime Player 10.x: Shorten a movie or a movie clip
Russ