You can use both Motion 4, Motion 5 and FCP 7 on the same machine however there are a few things to consider.
Motion 5 is designed to be used with FCP X and while it will work as a standalone it does not directly interface to FCP7.
You cant simply "Send to Motion" from FCP7 and heve the project automitically open in Motion 5.
So there is no round tripping and Motion 5 projects can not be opened/ imported to FCP7
I think you could use send to Motion (4) - open that in Motion 4 - save and then open that saved Motion 4 project in Motion 5
You would then have to export out a Movie from Motion 5 and import that into FCP7.
This is fine but what it does not allow for is making tweaks in a Motion Project that are then automatically updated in FCP.