Although you can sometimes open a version 4 file in version 7, the program isn't designed to work that way. Because it undergoes such major rebuilding with each generation, it's recommended that you don't try to open an older version project with a newer version of the software.
Even those who can have sometimes seen strange behaviors in the program when they've done it.
As Steve points out, sometimes it works, but sometimes it does not. It is always a good practice to finish a Project in the version that it was created in. This is not just for PE, but PrPro, as well. Now, I have opened much earlier versions of Pr files in PrPro, and even brought in Projects from Mac, to PC, there is no guarantee that it will work.