Koss still sells a version of the Pro 4 headphone. Have you looked into replacing yours with a new pair. They retail for less than $100.
Also, if you are set on repairing your 4AAA phones, check on eBay because used Koss Pro 4 headphones show up there all the time. You should be able to snag a pair of used Koss Pro 4 headphones you can use for parts for under $50.