I run at 1.296 as the max, with Vrm 1.920. This is for running it at 4.5ghz. You just want to run it at 4ghz, so you might get by with something closer to 1.2. But you have to try.
Let's say you try at 1.2, and the PC runs, then you run a stress-test, and it still works. You could then try 1.15, for instance, and keep lowering it until it crashes. When it does, try increasing it by 0.1 instead. And the other way around if 1.2 does not work.