I would expect anywhere in the region of 3.8GHz to 4.0GHz on a good watercooling loop.
Do remember that just cranking up the CPU Multiplier on the 940 Black won't give you the best results. To find max CPU frequency, you can start by using Multi only with minimal base HTT adjustment and slowly work out where it's limits are. Once you have a vague idea of what voltages the CPU needs at given clockspeed ranges, start working towards dropping the CPU Multi and raising the base HTT. Also start playing with the Northbridge Multiplier as well in order to reach nice and high HTT and Northbridge speeds. The sweetspot for the Northbridge Frequency is typically anywhere in the 2400-2800MHz range. I also found the K10 architecture, like K8 to perform better with tighter memory timings so give your memory a good grilling as well once you're done with finding the ideal CPU parameters.
Let us know how you get on