I've found most often that those types of variables are usually due to either the miner (see if you have the same result using Claymore) or the underclocking/memory settings. Try setting all to default (# out any memory or powertune/core settings) and rebooting, if it's still the same change the miner to Claymore (globalminer claymore) and reboot. If it's still the same, check your risers and PSU and make sure you are providing enough power. It's also possible that you just randomly have it set up so every 4th card happens to just be better (better memory or clock speed). You can mess with the settings for each individual card to drive up hashrates once you are sure everything is stable.