I haven't actually got the game yet (hopefully it will be delivered to me this Friday), but I can say I'm making big plans to move my mapping onto Quake 4. I haven't released a map for over a year (since this one, which was the beginning of July according to my trusty text file), but I've still been working on and off on un-completed projects that continuously give me practise. However much like Quake 3 Arena's multi-player Quake 4's is, it is a whole new opportunity, and because of the fairly small number of multi-player maps shipped with the game, players may rely on new maps to continue enjoying the game. I hope mappers in general can improve everyone's experience of the game (not forgetting major modifications too), because that's always been a big part in the Quake games.
I'd very much like to see other current Quake 3 Arena mapping faces taking it onto Quake 4 too :-)