Yes, the problem is that it expect TWO FANs. To overcome it, you can connect RPM pin from FAN1 with FAN2 on control board. Just connect pin with yellow wire with the same position at FAN2 position. Always do this when disconnected power - next pin is 12V which will permanently damage control board when connected to RPM yellow pin. (I did it.)
For the time being you are not supposed to use Braiins OS with the R4 at all.
Oh sorry, I forgot one much easier workaround. Just simply add any 3pin or 4pin CPU fan and leave it on the top of the miner running freely.