Eight subtitle tracks…I have a very strong feeling that’s where the trouble lies.
I’d guess that if you did a SaveAs to create a duplicate of the project, then deleted all the subtitle tracks that this would open fine in CS4. And there may or may not be a number of subtitle tracks that come over from Encore 2.0 without error, but I don’t know what that number would be or if it is even greater than one.
If you did the subtitles in a script editor, you could try importing them into a CS4 project.
But unless you want to do a lot (and I mean a lot) of trouble-shooting to get to the root of the problem, I’d suggest you not bring that type of project forward from Encore 2.0 to CS4.