Yup. Have been done before and it worked well
[Linked Image from i.imgur.com]

Voted for 4M, though it's difficult for me to guage how far it is - maybe a little less, maybe a little more. I feel only those who invested in stealth should have a good chance to sneak up to an enemy from behind. "hearing" circle would allow non stealth characters to still utilize stealth at distance, while giving actual benefit to investing in stealth.