I dont think there would be any problem.I am using both the versions and they co-exist fine
The only issue, of which I am aware, was with CS2 Production Studio and the addition of PrE 4. Obviously, Adobe did not anticipate that folk might have the former, and then install the latter. When I did, I found that there was already an Adobe KB Article on the problems. PrE 4 broke most of the CS2 Production Suite, but a simple Repair Install took care of it. I was up and running in about 1 hour (after I found my CS2 Production Studio discs).
Now, two things to take into account: CS4 is still current and PrE 8 is new. The majority of users would be going from PrE to PrPro, and not the other way around (as I did with PrE 4). Reports might not have come in yet, if there are issues. To date, I have seen none in either the PrE, or the PrPro forum, and I read almost every post in each.
If there is an issue, I would anticipate that the old "cure," of a Repair Installation, would fix things again. However, after the CS2 vs PrE 4 thing, I believe that Adobe has thought about all situations with CS4 and PrE 8, but could be wrong.
Good luck, and sorry that I do not have the definitive answer for you,