ffmpeg conversion from *.MOV to *.MP4 not possible?

When I want to convert with ffmpeg (under 64bit Win7) a (digicam) *.MOV video into an *.MP4 video then this seems to be NOT possible.

I got errors like:

[mp4 @ 000000000213cec0] Tag XVID/0x44495658 incompatible with output codec id '13' ( [0][0][0])

and

Could not write header for output file #0 (incorrect codec parameters ?): Invalid data found when processing input

Is this really not possible?

Thank you