When I was still with the R5 1600X I could not rise above 1533MHz stable.
My motherboard and MSI X370 Gaming Pro Carbon.
I thought my mother board was the limiting factor, because it was T-topology.
So now I bought the 3900x and my gskill royal 4000 cl17 memory and it funcinou first to its native xmp.
This proves that the motherboard is almost nothing as a limiting factor when it comes to frequency.
I got better results with 1: 1 IF 3733mhz.
Below are 2 screenshots with 3733mhz and 4000mhz.
I just tuned the primary latencies, I'm really looking forward to the 1usmus Dram Calculator tool.
He has done a great job.