Isn't your problem that you are re-encoding the video, which you'll be again encoding again as you edit it?
I guess the best option would be to take your mp4 and create an uncompressed version with which to edit. That way, you'll only be encoding it once more afterwards, and hence the quality dip won't be as bad.
Remember that re-encoding lossy-compressed video over and over is like copying a video-tape... for each copy, you lose a little of the original.
That's why, if you can, work with uncompressed. - you might want to get yourself a bigger HDD ;-)
I guess the best option would be to take your mp4 and create an uncompressed version with which to edit. That way, you'll only be encoding it once more afterwards, and hence the quality dip won't be as bad.
Remember that re-encoding lossy-compressed video over and over is like copying a video-tape... for each copy, you lose a little of the original.
That's why, if you can, work with uncompressed. - you might want to get yourself a bigger HDD ;-)