If you want to convert MP4 files to AVI, you can go to try Kigo video converter free for Mac.
Kigo Video Converter for Mac supports converting all popular video formats including flv, MPEG1, MPEG2, MP4, 3GP, 3G2, MOV, AVI, M2TS, MPEG TS/TP (for HD Video), M4V, etc with perfect output quality and high conversion speed for Mac OS X users. You can also extract audio from videos.
The free video converter offers various powerful functions to edit your video files, such as trimming movies into segments, merging files to single file, cropping video size, setting video effects, and even adding watermark onto your movies.
If you want to know more, please visit http://www.kigo-video-converter.com.