But if you are going from LR4 to CS5 (or anything), if you let Lightroom render the file, then you are using ACR7.
When you 'Edit in...', it doesn't take you to the ACR sub program in PS CS, it renders the file into a TIFF or PSD and takes you right into Photoshop.
If you choose to 'Open as Smart Object', then there might be an issue.
As for the compatibility issue, I haven't looked too much into this one...but I can't imagine that Adobe with not make ACR7 compatible with CS5. They do limit backward compatibility, but it usually goes back at least two versions.