I just upgraded to Lightroom 4 this week and am haing trouble getting CS5 to open the Lightroom-edited photos. For example, if I adjust the exposure, clarity, etc. in LR 4 and then hit command E to take the file over to Photoshop for further editing, CS5 isn't opening that edited file. It is opening the original file that was imported into LR 4 but not the edited version. These are RAW files.
Interestingly, all of the older files that I originally edited in LR 3 (they have now been moved into the LR4 catalog) will still move over from LR4 to CS5 when I hit command E. Only the files that were imported from my camera card directly into LR 4 aren't cooperating.
When you hit Cmd + E - or right-click the photo and select Edit in CS5 you should get a dialog box that gives you two options:
1) <Open anyway>, and
2) <Render using Lightroom>.
If you select (1) CS5 will open the original without Lr adjustments,
if you select (2) Lr will send a PSD to CS5 with all your Lr adjustments.
If you don't get this dialog because you have suppressed it, go >Edit >Preferences >General tab, and under the section "prompts" click on <Reset all warning dialogs>.