Flash CS4 “failed to open” document error

I was getting this corrupt error all week from Flash CS4 file I had worked on about 2 months ago.
The error wouldn’t even let me open the file, let alone look at debugging it.
I had a few backed-up versions of the file, and all of a sudden they were all corrupt.
I also tried opening the file on other computers with CS4.
I tried installing updates but after searching online for a while I was no closer to finding the solution.

Solution:
I remembered I may have been using CS5 to create the original files, as I was using a different computer. So I downloaded a CS5 Trial and the file opened fine.