I currently use CS3 but have a version of CS4.
What is stopping you from trying it out? CS3 and CS4 can peacefully coexist on the same system. Given your decisiveness shown in the past, only you can decide and you have the means to make a decision. OTOH, I would not be surprised I you only make your decision when CS5 has been on the market for over a year.
Your question has about the same value as someone saying:
"I have a Beamer 530 from 2004 and a Beamer 530 from 2009. Should I be driving the 2009 version?"
If you switch to CS4 you will need the latest version of the Cineform software as well.
The CF suites for CS4 are not completely finished at this time, but are available and fully functional to do CF projects in CS4.
With Kona Bob's suggestion, I'd say that the time was about right. Make sure to update to the 4.1 version. Also, if you run the programs on the same system (a good idea, until you shake out any workflow problems), be aware that if you then unistall CS3, it will take a few parts of CS4 with it. A Repair Install of CS4.1 should fix things. Adobe has a KB on this, but I could not find the bookmark.