Is it possible to have InDesign CS5 save as a CS4 file by default?
I ran into this problem before, and from what I found, InDesign CS5 cannot save files directly in CS4 format. If you need to open a CS5 document in CS4, this page tells you how to save it in a backwards-compatible file. The problem is it's a read-only format (I think), which is frustrating when you need to bounce between different versions.
There might be a third-party utility to do what you want, though, so let's wait and see if anyone else knows about that.