I don't know when your license expired, but the last of the previous licenses expired in early December 2008. A license purchased now will be good for all future 3.x and 4.x releases.
(Which means, yes, you'd have to buy it again when it goes to 5.0. My goal would be to make it worth it, of course. Based on past experience, 5.0 will ship in 2012. That's not a promise, though, just a guess.)
-Brent