Well ... MPEG4 video is nunmal a final format and actually not really meant for further processing. For AviSynth:'s installed, open Notepad (the Windows text editor), pure read the following (of course with adapted path):
Save it as a starting Beliebiger_Dateinamen.avs (NOTE: to the eighth *. avs extension). This AVS file you open now instead of the MP4 file into VirtualDub (if the MP4 file can be played with WMP, it can process the kind AviSynth and share pictures and sound to VirtualDub). I just hope that it works Sun But as I said with 'sample would NEM's easier for me to give more detailed advice. That you that's too sensitive for a private video, but I can understand - but it runs just the way to more "trial and error" out ...
DirectShow source ("C: \ Pfad_zum \ video.mp4)