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
Is this firmware
Antminer-R4-xilinx-201811121649-autofreq-user-UBI-NF.tar.gz the one with ASICboost enabled, or is it another one?