To be honest I'm not comfortable about UrT 4. It's standalone in the sense that it claims not to use any Q3 assets. However, there are things that are directly copied from the Q3 pk3s. I think the reasoning behind this is that it's permissible to redistribute Q3 assets with Q3 mods, which UrT still is. I'm unclear if this is still OK when uses in a standalone context.
The code is all still closed source and it isn't convenient to install as it needs a separate "engine" installer. The point of this is basically to evade the GPL as far as I can see. This much is all legal I think, but only just. The one area where I'm not so sure about is with the Q3 EULA on which the mod SDK is bound (and thus the closed source UrT), which states that it may only be used with "Quake III Arena". I'm not convinced that the ioq3 fork that is distributed for UrT still constitutes "Quake III Arena".
It's all a bit grey and murky in my opinion.
