Also after sorting out overclocking stuff I'm getting this annoying bug that 1 card doesn't get loaded properly (had it with ewfb's under w10 too occasionally).
Since all the other card are constantly on 100% it seem that it isn't an cpu performance issue - however check your cpu-load to be sure. Most likely it's a pcie-bus issue, the bus that GPU4 is connected to seems to have latencies. If you're unable to find the reason for this, I suggest you to swap the GPUs and put the slower 1060 into the pcie-bus that GPU4 is currently on.