I wouldn't bother waiting for 4 confirmations unless the transaction was in a four-digit range or higher (if it was a 5-digit or higher one maybe you should wait for 6 confirmations).
I'm not sure about 0-conf transactions but 1-conf should be enough for anything below $500-1k (so most PoS transactions).

You should first ask yourself a few questions:

Who is able to do double-spends?
If they are, how much would it cost them?