I also had the same problem. Here is another solution that works quite nicely but requires QT Player 7.
TL;DR Use QT Player 7 to trim the first frames of the mp4 file and export with codec of choice.
Step-by-step Instructions
Open the mp4 file in QT Player 7. Move your playhead a few frames forward and set an in point. (Shortcut key = i) Then move your playhead to the last frame and set your out point. (Shortcut key = o). Then “trim to selection” (Edit/trim to selection).
After you have trimmed the selection, Export the file (Command-E) and under the export settings pick your codec of choice.
Easy Peezy Lemon Squeezy.
Cheers,
Octavio