Q4Max adds features which make Quake 4 suitable for use in competitions, fixes and general improvements to the game, without changing the underlying gameplay. As such, it has been used by all the major eSports competitions - CPL, ESWC, WSVG, GGL, CB, QuakeCon, and more. Over 50% of the online players for Quake 4 at any time are are running q4max.
In addition to simply making the game slicker, it features a number of new gametypes (Duel, Clan Arena, Capture Strike, and Freezetag - both US and EU variants), as well as features such as the ability to enable an Instagib mode, allowing server operators to configure the server the way they want.
Q4Max includes an extensive menu which allows every option that players are likely to wish to change to be edited without changing .cfg files, or cvars at the console.
There are many other changes - too many to list here. The last release alone had over 70 separate changes, and was the 8th major release over the last year since the Quake 4 SDK was released.