Probably not. It has never been possible in previous versions to import a higher version project file backwards.
Maybe another solution available depending on what you are trying to do????
I have had some success in opening sequences from a CS5 project in CS4.
I fired up CS4, created new project, file -> import and selected the CS5 project. Premiere asks you if you want to import entire project of selected sequences. I selected a sequence and found it imported just fine into CS4. Note, however, that these were very simple sequences, with no effects and straight cuts or dissolves. I've done this a few times now, and it seems to work.
Interesting, helpful observation, Mark--thanks for posting it.
If that doesn't work for the OP, he might consider exporting an AAF file from CS5. Go to File > Export > AAF and follow the prompts. AAF is a sort of "universal" project file format--at least, it's as close to universal as we're likely to get at this point in time. It should do a pretty good job of maintaining most edit decisions in your CS5, as well as effects and imported elements like PSDs and AE projects. I just did a quick test on a basic CS5 project, and it came into CS4 without a hitch, near as I can tell. Just import the AAF like any other file, and Premiere will do the rest of the work.
I'd say that if Mark's suggestion works, then go with that--but if it doesn't work with your particular project, check out the AAF option.