It really depends somewhat on your workflow. Lightroom and Photoshop are becoming very close to each other in what they offer for photographers.
If you go out and shoot and then do basic adjustments to your images then Lightroom would probably do just fine. If you like to go in to your images and do things like skin softening, significant cloning to fix things in the image, etc. you would find that Photoshop is required.
Everything that Lightroom can do can also be done by ACR 5.x which ships with CS4. The advantage that Lightroom provides you is the nicer looking interface and the database. If you do alot of keywording it is easier to do this in Lightroom though it can also be done with Bridge. You need to look into the future a bit. Are you getting a large collection of images. If you bring them into Lightroom and develop yourself a folder system it will make it much easier to find an image 6 months later. It has search options that none of the products that come with CS4 have.
If you can afford both I would certainly suggest you get them. I think they are a great team together. I know that alot of photographers are saying that they have reduced the time that they spend in Photoshop and that is a good thing. The improvements in CS4 are worth the upgrade price, imo. Things like the live preview makes cloning a whole lot easier but it does take some horsepower in your computer. I can do it quite easily on my laptop which only has 1GB of RAM and not that fast of a processor.
