There is no compatibility problem with having CS4 and CS5 on the same machine, so that's not your problem.
Whter or not your license allows it depends on your type of license (my understanding is that some volume licensing prohibits it) and how you interpret transition period.
Well currently, I'm just testing this issue. I just installed the trial version of Adobe CS4 suite on my mac and then deployed the CS5.5 package on top off it The resulting errors that I listed above are the same ones that occur when the end user has a licensed volume license install of CS4 and CS5.5.
So I'm still back to square 1 where when I deploy cs5.5 on an end user's mac, they get those plugin errors when they launch Indesign CS5.5.
I am not sure about the licensing restrictions, I asked my Director and he is going to check with our VAR.
I think the problem is in the deployment package you are using to install CS5.5. There are lots of us who have multiple versions from CS5.5 on back. I persoanlly go to CS on this machine, on XP I went all the way back to version 2.
Well we are using recent Intel macintoshes with 10.6.8 snow leopard fully updated.
I'm going to rip both cs4 and cs5.5 from my test computer and then install cs4 and then install cs5.5 on top of it but not using the deploy package and test.
It really just sounds like you corrupted those plugin files.
If you had a licensing problem, you would get a licensing error.