How to unlock the ETH79‐X5 motherboard for minimum 4x NVIDIA Tesla P40?

Some background

The ETH79-X5 mining motherboard is designed for usage with 5x 3060. These GPU has 12 GB of VRAM which sums up to 60 GB VRAM in total. The motherboard works fine with 2x Tesla P40 out of the box since 2x P40 have 48 GB of VRAM but when a third P40 is installed the total VRAM would be 72 GB which causes the board to not POST anymore.

What to do

Running 3x P40 is simple. Just change the MMIOH Size in the BIOS to 128 GB. But to run 4x P40 we need a little hack.