While not the strongest entry in the series, Saw IV was still a well thought out movie. Expect some more inventive "games," a fairly interesting (if a bit drawn out "enough already" Jigsaw backstory), and enough gore to satisfy you (if you like that sort of thing). By film's end, you know they can keep going, although I'd hope they'll slow down because one a year is definitely straining the writing a bit.
Also, (and I don't feel this is a spoiler) be prepared to see prior Saw series events in a new light and expect to be a bit flexible on your chronology for the franchise. If this is a spoiler, I apologize, but it seems to me that it pays to see these new games as taking place simultaneously with the games in Saw III...you'll see what I mean when you watch. (There may still be a few flaws here, hence my plea to slow down the annual releases.)