Are you running the latest, ASICboost enabled firmware? My R4 would drop a board often enough, and rebooting usually got it back up and running, but after updating the firmware to the latest it runs better and hasn't dropped a board yet.
HTH
It was on the ASICBoost FW, then I rolled it back to see if that helped (it didn't). Not sure if I put it back on or not.
-Dave