There's not really enough info for anyone to help you.
Please read the Sticky at the top of this forum to get the fastest help.
AVI is a container, not a codec.
MPEG4 is too vague without settings details.
Playback doesn't mean enough without a player and a version.
Mac OS doesn't include version numbers of version specific issues.
With the information provided all I can offer is that there's an issue with your settings or your player and maybe the codecs you have installed.