Sounds about right to me. 3.8-4.0GHz is about the norm with air or water with the AM2+ Phenom II's. Supposedly the Phenom II X4 955 AM3's been doing better but naturally at a higher price. He might be able to squeeze more out of it if he's man enough to push more volts through it, but I'm not sure how much further he'll get. If it's one thing I learnt about Phenom II chips, they seem to appreciate cooler temperatures.
With the Phenom II X4 940 Black going EOL, you may start to find some really good deals on them as retailers try and clear stock. Cheapest I've seen them is £144 + Delivery and £150 picked up from the retailer near me.
By the way, SuperPi isn't impressive with Phenom II. AMD's CPU's seem to run particularly badly with the app.