Your best bet when buying off Ebay when it comes to price would be to find one with a broken disk drive - you'll be removing it anyways so it doesn't matter.
Regarding finding a 4 layer, read the basic identification guide that Aurelio previously linked you here: https://bitbuilt.net/forums/index.php?threads/revision-identification-guide.863/
If you are still having trouble finding a 4 layer at a decent price then you may want to look at this thread where there has been effort to corrolate the serial number to the board revision. Mind you this method is not a 100% surefire method: https://bitbuilt.net/forums/index.p...iis-revision-based-off-of-serial-numbers.716/