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?
That is the ASICBoost FW from Bitmain. However, if you are not on the other listed firmware:
Antminer-R4-all-201704280718-autofreq-user-Update2UBI-NF.tar.gz
The update will not work. The update listed above changes the file system to UBI or something like that which is necessary for the AB firmware to be compatible as it uses a UBI filesystem but does not contain the necessary files to changeover the existing filesystem.