Sure you can use both on the same system. However, I would rename Motion 5 to something like "xMotion" because if you don't, when you round trip from FCP 7, it might make Motion 5 launch instead of Motion 4. This way, Motion 4 will still be the default app.
--
Mark Spencer
Freelance Producer/Editor/Motion Graphics Artist
Apple-certified Master Trainer
Author, Motion 4 from Peachpit Press
http://www.applemotion.net