3.12.0
As I feared. Your S4's are working fine - it's just that someone is stealing your hash.......
Bitmain fail.
Can this be prevented in any way in the mean time? Use of a proxy, or something? Why doesn't this happen with all pools?
Not that I'm aware of & it can.
If you put a proxy between your miner and the pool
locally on your lan, and ONLY point your S4 at the proxy, you should be good to go and it will prevent it being redirected. ckproxy (linux only) which supports failover as well can do all the failover work for you if need be. Link to ckpool/ckproxy code in my sig.