Stamper, recently I purchased LR4 instead of upgrading from PS-CS4 to PS-CS6. Iíve also read Martin Eveningís LR4 book. So, Iím far from an expert, but it appears that exporting a photo from LR to PS for additional editing, using LRís rendering, gives essentially the same result as if you had CS6. The difference is that, if you did have CS6, then PS could use ACR7 to process your RAW file (using your LR settings) and then open from ACR a tiff version in PS (which can be a smart object).
The only real difference Iíve discovered is this smart object difference. I canít open my LR files in PS as smart objects. So after I make some layer edits in PS, I canít just open the smart object to tweak my LR settings. Instead, I have to tweak my original LR version, and open it in PS, drag it onto the layered version and use it to replace the background layer. The end result is identical, but not as quick or easy as using a smart object. Itís not a big deal for me since I donít process a lot of photos, but others may find it a much bigger hassle.