MPEG-4 Encoding Guide
MPEG-4 Part 2, aka MPEG-4, Xvid, and DivX, is a video codec that was most widely used before the wide adoption of H.264. FFmpeg has two encoders to output MPEG-4 video. The external encoding library libxvid:
ffmpeg -i input.avi -c:v libxvid output.avi
...and the native encoder mpeg4:
ffmpeg -i input.avi -c:v mpeg4 -vtag xvid output.avi
The native encoder has the advantage of not requiring an external library. Both encoders should provide a similar output, but for lower bitrates/quality (e.g. 1000 kBit/s for 720p content), libxvid will deliver better quality than mpeg4.
The default FourCC stored in an MPEG-4-coded file will be FMP4. If you want a different FourCC, use the -vtag option. E.g., -vtag xvid will force the FourCC xvid to be stored as the video FourCC rather than the default.
Variable Bit Rate with qscale
You can select a video quality level with -qscale:v n (or the alias -q:v n), where n is a number from 1-31, with 1 being highest quality/largest filesize and 31 being the lowest quality/smallest filesize. This is a variable bit rate mode, roughly analogous to using -qp (constant QP [quantization parameter]) with x264. Most of the time this should be the preferred method.
You can select an audio quality level with -qscale:a (or the alias -q:a). The value changes depending on the audio encoder. Since this guide uses libmp3lame see the MP3 Encoding Guide for examples and more information.
ffmpeg -i input.avi -c:v mpeg4 -vtag xvid -qscale:v 3 -c:a libmp3lame -qscale:a 4 output.avi
There is rarely a need to use -qscale:v 1. Note that if you choose it, libxvid will take much more space than the same video compressed with the native mpeg4 encoder.
Constant Bit Rate
You can target a bitrate with -b:v. This is best used with two-pass encoding. Adapting an example from the x264 encoding guide: your video is 10 minutes (600 seconds) long and an output of 50 MB is desired. Since bitrate = file size / duration:
(50 MB * 8192 [converts MB to kilobits]) / 600 seconds = ~683 kilobits/s total bitrate 683k - 128k (desired audio bitrate) = 555k video bitrate
ffmpeg -y -i input.avi -c:v mpeg4 -vtag xvid -b:v 555k -pass 1 -an -f avi /dev/null ffmpeg -i input.avi -c:v mpeg4 -vtag xvid -b:v 555k -pass 2 -c:a libmp3lame -b:a 128k output.avi
Note: Windows users should use NUL instead of /dev/null.