You can still use LW 4.18.8 Pro on your system. But you can try sending a PM to limenathan (click on link). I'd think it might be doubtful however for a couple reasons. One is it would cost them to cancel the transaction. Secondly, it's against their normal procedures.

LW 5 is not only limited to OSX 10. 5 or later, but also limited to 64-bit intel machines. I guess that's one point you can also add in your support that there was no notification that LW 5 was limited to intel machines & to Leopard OSX.