Vlad dropped us a line to say that MapMixer has been updated for UT2004. What is MapMixer, you ask?
MapMixer is a Mutator for Unreal Tournament 2003/4 that gives you extended control of Maps, Game Types, Mutators, Player Counts and more. MapMixer replaces the standard map list with it's own system and takes control over the options that maps are invoked with. Think of it as an extension to the way you use UT2003. MapMixer can be used in a single player game or in a network game with server and client support.
The mutator is still an alpha release, yet it is fully working and functional. It can be downloaded here.