Maybe you need diffrent OS

I also have rig with 4 reference Vega 56 (flashed with 64 bios).
Here my best results with diffrent OS and BIOS:
TeamRedMiner 0.4.4 under MMP OS 2 (Ubuntu)
4xVega 56(stock bios)
core 1200mHz/850mV memory 910mHz/850mV
7.760kh/s @ 640W from wall