There are some significant changes. Some things that were fairly common houserules (HP based off of ST and Fatigue off of Health instead of the other way around, for one example) are now official. Attribute costs are now linear, and DX and INT cost double. The biggest changes are in the way powers are integrated into the basic system instead of getting tacked on. Some things update easily, others don't. I'm pretty sure anything that was in the White Wolf conversions can still be done in 4e, but you'd definitely need to convert it yourself, you could not use it as is and they ain't getting that license again, so no official update. I feel it's a worthwhile upgrade, despite having 40+ 3rd edition books (I'd feel more that way if I ever actually played the game any more. Sigh) but YMMV. I will say that as far as I know, all the GURPS games on the Weave are 4e. Might have something to do with the small (but quite active) number of players.