MP4 isn't widely recognised as a vid to edit which is exactly as it comes out of the camera. You should convert it to a MOV file and this should be fine to edit it. With mine I have converted to a MOV file using an Appleprores codec, edited it in Final Cut then exported it from Final Cut back to an MP4 file and into Quicktime to convert to an avi file....phew still with me

I've tried 1080 footage and not had a problem running it or editing it but I have converted it first. Before then it was a nightmare. That said I'm running it on a Mac rather than a PC.