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.
The CaptureStrike (CTFS) gametype is a cross between Clan Arena and Capture The Flag. Unlike CTF, both teams don't attack at the same time - the game is played in rounds, and they take it in turns to attempt to capture the flag.
In this screenshot, you can see a player about to capture the strogg flag, while the marine flag is grey to indicate it cannot be captured.