I have 10 S4 running.
All stable at 2 TH/s, no problem with the firmware our psu.

My S4 is also stable at 2 TH/s as well.
Did you update cgminer from 3.12.0 to 4.6.1?
Truman wrote in the german section that he is mining on BTC Guild...
What is your pool?
It is solo.ckpool.org, but after updating the firmware, the miner status goes higher, but ckpool has gone lower.
In this picture it shows 3576.94 GH/S(5s), does that mean it went up to that for five seconds?
because since the firmware update the (5s) part seems to bounce up quite high? it didn't do that before the update.
http://i59.tinypic.com/1z4ctic.jpg