I have same 4 ASUS 1060 3GB, all OC, but one works slowly then others.
ETH: 05/31/17-02:41:36 - New job from eth-eu1.nanopool.org:9999
ETH - Total Speed: 78.055 Mh/s, Total Shares: 7, Rejected: 0, Time: 00:06
ETH: GPU0 17.358 Mh/s, GPU1 20.249 Mh/s, GPU2 20.263 Mh/s, GPU3 20.185 Mh/s
GPU0 t=47C fan=75%, GPU1 t=52C fan=75%, GPU2 t=51C fan=75%, GPU3 t=48C fan=75%
What are the complete specifications of your system, to include riser type?
Have you tried swapping out the riser on that card?
How are you powering your risers?
I have old asrock 970 extreme4 with 4 ASUS 1060 3G. USB powered risers Blue 006C. In ZEC mining mode cards work with same hashrate, but ETH