Its right at the top of the page in AI tweak page thingy.
I think I may have sorted things even further - It seems that perhaps one of my memory modules were bad too... After posting the above, i tried a little more the 3.9ghz and it crashed. Restart and reset bios and it would crash under benching, even at stock speeds. I noticed one of the memory modules was a little warm compared to the other, so pulled it, and tried with 8gb... solid 3.9ghz. thought I would push my luck and chuck in a cosair value 4gb module, a 2133mhz one, and try and run it with the other at 2400mhz and the DOCP timings - weirdly, it worked! so it might be that this other module was bad.
Funny though how it has been working for year+ in my other system. It seems ryzen is very very very sensitive to crappy modules (doesnt seem to care about mixed modules though)....
All very strange this...