Q: el capitan - numbers, pages can't open files anymore

So when you say 4.0, you mean 6.0? and 5 days ago you mean 7 days ago? and Adobe CS6 you mean Adobe CS8?

Or have I got something wrong here?

Peter

You are attempting to open a Pages v5 or v6 document in Pages '09 via the Open file chooser. It will tell you that you need a newer version of Pages, but in reality the v5 and v6 documents are simply incompatible.

Actually quit Pages '09 from its application menu, and then just double-click the same document(s) that would not open in its open chooser. These will then open in the most recent version of Pages v5 or v6 on your Mac.

Just to clarify, Pages 5.6.2 & Pages 6 documents are compatible with each other. Pages 4.3 has never been able to open Pages 5 files & never will unless you export as iWork '09.

I should have said, “in reality the v5 and v6 documents are simply incompatible with Pages '09,” as that was what I intended to imply.

I knew that, I just want to make it clear to the OP.