I'm sorry, but I don't recall that AE CS5 was able to backsave to CS4. They were two very different applications. CS5 was 64-bit, and CS4 was 32-bit.
Dave is correct. See this page: opening After Effects projects from previous versions and saving back to previous versions
Now, there is the old OpenSesame trick that might or might not work. pt_OpenSesame It claims to work all the way back to CS3. One major issue is that CS4 was when AE was a 32-bit program and CS5 is when it became a 64-bit program, so you have to watch out for that.