I'd try unlocking the cores and then OC'ing... seeing as there are plenty of multi-threaded applications now to make use of the 4 cores. It shouldn't make too much of a difference though in your overall clock speed. Like you said... maybe 4 GHz to 3.8.
I had a X3 720BE, and if I unlocked the 4th core, I would need to increase the voltage a tad, but nothing major. You'll just have to try the chip out for yourself, you might not even be able to unlock them.
Overall though... I'd take the extra cores with the minimal (if any) decrease in clock. I'd rather have a X4 @ 3.6 than X2 @ 4.0