there are a couple of differences we have to clear up here first:
a fla 5 file
and a swf movie created in flash 5
are of course diff.
now saving a fla 5 file and opening it up in flash 4 on any other computer will not work.
but publishing a flash 5 movie and opening it up in flash 4 on another computer may look weird and not work right, but it will still open.
i assume you are talking about the former...
so in that case in general fla 5 files cannot be opened with flash 4... and as soon as you open up a fla 4 file in flash 5 and save it again (overwriting the previous version) you will not be able to open it in flash 4 again.
sorry.
perhaps there is a utility out there that can change this fact... but i have not yet heard of such a thing.
have fun.
-aldorr