I didn't know PT4 was illegal anywhere. Seems unlikely since it is so popular, along with HM2. Looking at the PokerTracker website all of the sites below would seem to be legal. Certainly I've never had any issues using it at different sites.
As far as whether it's "worth it" that depends on how technical you want to get with your game IMO. It is a very comprehensive tool that has a learning curve. You are getting your money's worth if you're the kind of person who wants to use a fancy and powerful tool. I don't use it anymore because I play mostly Zoom poker at
pokerstars and I find that I do better relying on my own observations.