Despite its success and popularity DivX 3's legal status was questionable. This led to the development of DivX 4, which was based on the work from OpenDivX (ProjectMayo). DivX 4 has no relation to DivX 3 except for the backward compatibility.
DivX 4 uses the fourcc DIVX (as do its successors DivX 5, 6 and 7). DivX 4 can decode DivX 3 contents (with occasional glitches).
Since the DivX 7 codec can decode DivX 4 contents, but not vice-versa, it makes more sense to have DivX 7 on your system. DivX 7 can be downloaded here: http://www.divx.com
Users of Windows 95, 98, or Me, won’t be able to run DivX 6 nor 7 on their system, but they can get DivX 5 here instead (get DivX 5.2.1 (not Pro) for Windows 98/ME).
Should you (for whatever reason) wish to encode in DivX 4, you can get version 4.12 of the codec here http://www.divx-digest.com/software/divxcodec4.html
(keep in mind installing it will overwrite the Divx 5, 6, or 7 codec if you already have it - you can’t have them both on the same system at the same time)
New: Windows Media Player 12 in Windows 7 can play DivX contents without any additional software.
Notes: XviD and 3ivx can also decode DivX 4 contents.
Keywords: DIVX, divx, DivX 4 
posted in Solutions