Is there any point to using Lightroom 4 if you already have CS4 or later? Not necessarily purely because you can already do everything that Light room 4 does from your CS4 (or later) and much more. However, having said that Lightroom 4 does present a more user friendly all in one application that integrates all the various elements from the different and vastly more extensive individual apps within the comprehensive CS4/5/6 suites that are relevant to the specific purpose of photography and managing your photograph catalog. Generally Lightroom 4 gives you a more streamlined/lite solution that’s solely dedicated for the purposes of photography and nothing more. But here's just a few features that you might find useful...

Powerful picture management - This comes as standard in Lightroom 4 much in the way that you would use something like Adobe bridge in CS 4/5/6. There's also a feature to call up predefined current project folders for faster access along with a boatload of other powerful picture management stuff.

Immediate image pre-edit features - There's also the ability to immediately tweak images from within the larger picture view pane whilst reviewing thumbnails in Lightroom without need to load a separate app like Photoshop to do it.  Generally in CS4/5/6 the workflow would more likely entail three steps of you needing to scan through and select in Adobe bridge to then have to load up Adobe Photoshop to do the pre-edit tweaking/processing before actually getting to the fuller editing within Photoshop itself if working with RAW files.

Easy slide show creation - you can also quickly create slideshows from within Lightroom. With additional features that allow you to sync your slideshow to music as well as the exporting of your efforts to a video file. This would negate the need to use the full on Adobe Premier pro video editing suite and Adobe's after effects.  Although if you want to take it a little more seriously on the soundtrack front you might still need Adobe Audition (available as standalone audio editing suite or as part of CS 4/5/6) too.

Simple and powerful web page flash gallery creation - Lightroom 4 has a powerful and easy to use utility that allows for the creation website gallery pages using the many templates that they provide. After you've created it you can simply upload it to your website, no coding knowledge or heavy webpage design skills required. This would essentially negate the need to use the extensive features of Adobe flash builder and Fireworks in CS4/5/6 that have far broader in depth flash and web design applications then just webpage gallery creation.

Integrated social network picture management - Pretty much as it describes.

Essentially Lightroom 4 might be useful to have on a machine that you might be using for photography in combination with Photoshop (or something equivalent to Photoshop) that doesn't already have the comprehensive CS4/5/6 suite on it. As ever it's possible to try out Lightroom 4 to see for yourself if it something you'd find useful.