How can I set up GTK Radiant 1.4 for multiplayer maps ?
Well, all you need to do is go to File > Project Settings and set it to multiplayer. Even so, it isn't strictly necessary, but it does put all the entities in order and takes out the non working ones and I think adds a few MP only entities. (team related)
Other than that, everthing is the same. Make your map, script it if needed, added appropriate spawns for it, etc.
rather then that its best to do that, but some you can make a single player map work in MP, altho i've had the chances of many crashes cause of that mistake, certain triggers apply to certain maptypes, so good luck and happy mapping