I've been mining with p2pool lately and it's working perfectly.
Would you mind sharing the miner settings you're using with p2pool? Also, what is your reject/doa rate with the S4 & p2pool?
Thanks.
That's with 2 S4s running. Latest firmware installed (10-16) with ck's custom cgminer, each running at 225MHz. I edited cgminer.sh, changing queue from 8192 to 0, and added expiry 1 and scan-time 1. Share difficulty manually set to 2048.