Is the project HD or SD?
I've just started a fairly complex 1 hr HD program in CS3. I feel that we've only just got all the bugs out of CS3, no way I'm going to dive into something important with the new CS4. If you look at the posts in CS3 forum- they are mostly about technique, whereas many of the posts in CS4 forum are "CS4 crashes...", "BSOD...", "CS4 won't install...", etc., etc.
Also, I've quit trying to do long HD programs as a single project in PPro. I have had problems with Premiere getting slow and buggy as the HD project gets out to 30-40 min. in length. Now,I divide the program into "reels" of about 15 min each. Each "reel" is a seperate project. When finished, I export each reel to a "final assembly" project. Sometimes I will add the final music tracks at this point, then export the finished movie out to the delivery format.
If CS4 has some new feature that you just absolutely need, then go for it, otherwise you might want to stick with the devil you know.