I'm not sure about getting CS4.. I'm pretty lazy in the respect that I will let a few versions flow by before I upgrade... Then again the latest photoshop is not a deal breaker for life... just for my side hobby of drawing..
I have got the full Adobe CS4 Master Collection.. an I love it
it's not a very huge difrence but... in Ps one big difrence is the way you can handle with 3D images... for example you can take any brush and just start to brush your 3D modell and so on.. in Dw one big update is that you can "try" your website "live" inside Dw, like testing al buttons and menues and so on, actually realy handy...
Yes! I am working on the CS4 Master Collection... and I have to say that I am liking the workspace options. I feel pretty comfortable working on designs, wireframes, html, etc... for projects, and the thing I run into a lot is having multiple files open and switching back and forth with copying and pasting, dragging and dropping, and opening and closing.. etc. CS4 seems to really put a focus on the organization of workspaces (multiple artboards in Illustrator, tabbed file organization in photoshop, relative files in dream weaver and vertical split screen in dreamweaver with live preview, etc) and that is a great time saver... and if you make money by the hour... then that means a pay increase!!! So, I would say it is worth it if that is what you're looking for.
The only thing I hear that is the real downfall is that since the guy from HP has entered the game with Adobe... color matching for printing in Photoshop has been a real problem and that a lot of designers that work with print photos and other related print work are going back to CS2... but I hear that 4 has improved in that area... I will find out soon I guess.