I've used both of these programs extensively in a professional setting, and here's my opinion; CS4 is for the perfection of an individual image, or slowly batching images with very specific settings. It seconds as a graphics program, which also comes in handy. Lightroom is more limited in its editing scope but is more focused on handling thousands of images efficiently. It will batch out numbers of images much faster than you can with CS4 and Bridge. My workflow involved the basic editing of images in lightroom (exposure, color correction, contrast, etc.), as well as the conversion to Jpeg and cataloging. The images would then have actions applied in CS3/4 as needed for black and whites, etc.
It is true that they both serve different needs. If you are only concerned with the basics, such as exposure and color, and you work with a lot of images at a time, then Lightroom may be for you. And lightroom, along with presets, can render some awesome black and whites and a few other special effects. Photoshop on the other hand will be more painstaking if you have to deal with numerous images, but you can end up with much more dramatic results in terms of post processing (HDR, Pano-stitching, Composites and very dramatic color and special effects). Though, Lightroom 2 added much of this capability with brushes and such.
Like others have said, they are meant to go together in your workflow. If I had to pick one, it would be Photoshop because it comes with bridge which can serve some of the functionality of Lightroom's database and thumbnail editing.