The files that have been saved as illustrator cs4 will not open in cs6. What, if any, are the ways these files can be opened?
You have a few options, some could be more destructive than others but at least offer you the opportunity to save some of the artwork. Sometimes, the file is simply corrupted or lost some data during an uncompressed transfer.
You can try these options:
1. Change the extension of the file OR add one if it's not there
Some designers, especially on mac, don't add any extension. Or add the wrong one. So try using .ai, .eps, .pdf, .svg and try to open the file using the "open" in the menu or dragging the file on the software or right-clicking the file and selecting Illustrator. Not by double-clicking it.
2. Import that Illustrator file into InDesign and save it as a PDF
Import your old Illustrator file in a software like InDesign or QuarkXpress, then export it as a PDF. Then open that PDF with Illustrator.
3. If your version is really older than the file
Then you should try the option 2 or use a trial of Illustrator CC or a friend to resave the file to your version.
4. Maybe the file isn't really an Adobe Illustrator file
Because of the reasons mentioned in 1., you can get some clues about the real file type by changing the extension to .txt and opening it in the TextEdit software. You should see there what's the version of the file and how it was created if it's a vector.