As far as I know SPoG just got his copy of Quake4 last week. I asked him before he got it if there was going to be Q4 support in GtkRadiant and he told me in not so many words that there would be. I think he needs the SDK for it though. So anyway, hang tight and there probably will be. I know Im waiting for it because I use Linux and the in game editor doesnt work at all in Linux.
As an aside, does anyone know if the 'multiplayer' portion of the D3/Q4 engine can be launched on init, instead of whatever passes for the single player portion? It's a relatively minor thing compared to the other issues, but its annoying that the startup and init of multiplayer is so slow.