I have the X4 965b in my "daily driver" system overclocked to 4.3ghz and it's stable, although I could see where overheating is definitely feasible if I were to find some task that required the CPU to operate at 100% load for fifteen or twenty minutes with literally no dip in the processor load (not likely in real life; more of an observation from stress testing). Of course, I am still using the 92mm DeepCool push/pull cooling setup from my previous undersized case- I am sure if I had your cooler it wouldn't be an issue.
IMO, I think the amount of noise in your PSU's output has a lot to do with how high the bar can be set with this particular CPU. I really can't imagine that there are chips that are limited by the manufacturing process to 3.8ghz while others are okay at 4.5ghz with everything else being equal. When the processor is pulling 1.62v, I don't imagine that it would take much for a cheap PSU to nudge things over a cliff.