I tried numerous ffmpeg settings to convert some M4V (MP4) files into AVI. Most either didn't work or crashed with a segmentation fault.
The only one I found that worked was this answer at StackOverflow. However, the quality of the file produced using the settings in the answer wasn't great, so this is what I ended up using:
ffmpeg -i input.m4v -f avi -b 2048k -ab 160k -ar 44100 output.avi
The output files are about twice the size of the input file, unfortunately, but they look decent.