Q4Max adds many features and fixes to Quake 4, making it a much more enjoyable and complete game. For this reason, most servers running Quake 4 are running Q4Max.
In this screenshot, you can see a timeout being used, which allows players to pause the game for up to 1 minute. For referees, there is an additional "pause" command that allows an indefinite timeout, which can be used in the case of problems.
Notice how the hud is different in this screenshot to others - this is using the "superhud" to change the screenlayout. Players can change layouts between the supplied defaults using the "hud_style" cvar, and they can create their own hud configs if they wish.
You'll also notice in this screenshot the lighting is different - that is the "vertex lighting" mode. This may look ugly, but is preferred by many players, and also makes the game run much better on old and slow hardware.