If you want to rotate a mp4 video file in any direction here is a simple one-liner:
mencoder input.mp4 -o output.mp4 -vf rotate=1 -oac mp3lame -ovc lavc
Where rotate could be:
Rotates the image by 90 degrees and optionally flips it. For values between 4-7 rotation is only done if the movie geometry is portrait and not landscape.
0 Rotate by 90 degrees clockwise and flip (default).
1 Rotate by 90 degrees clockwise.
2 Rotate by 90 degrees counterclockwise.
3 Rotate by 90 degrees counterclockwise and flip.
For more information use man mencoder.