That's probably just some Turbo at work. Folding is too heavy for max Turbo to be reached, but it's not necessarily too heavy for a light Turbo frequency such as 4.1GHz to be reached, at least if you have a very good FX-8350 (not all CPUs, even of the same model number, are equal when it comes to silicon quality). Perhaps there may be other factors involved too, but reaching 4.1GHz is not necessarily strange.
It might be that you got lucky with your unit, but I'm not familiar enough with the workload to say that for sure. You could test this theory with some overclocking if you care about it, but I don't think that it really matters unless overclocking is something that you intended to do anyway. 4.1GHz versus 4GHz isn't going to make much of a difference anyway (it's only 2.5% higher in frequency).