I don't have 4E to tell you the answer.
However, I'd be surprised if there were rules. From what I can tell, D&D has never focussed on hit locations or specific injuries. So barring custom rules (say Dragon Magazine or another product), I don't think there really are any.
Part of it is that HP sort of represents your physical body as well as some measure of "avoiding serious injury".
If you get into specific and serious injuries, you can quite easily disable a PC beyond playability. Most players hate losing limbs, and if you "spared them a death" by forcing a limb loss instead, some number of them would have actually preferred death (of the PC).