It's a Pentium G3258, bonus clocks of 1.5 GHz are common, if not expected. On the stock cooler.
No, i mean is he selling you a KNOWN chip, as in tested @ 4.x ghz.
I have done 4x g3258 machines 4.8ghz 4.7ghz 4.6ghz 4.4ghz
The 4.4 means the spread is quite wide..
Oh I see what you are saying. Well, he claims he runs it at 4.6 in the winter and 4.4 in the summer without any issues, and this is on the stock cooler, so I expect (if he ever actually sells it to me) that I could push it further. Although I might have to look elsewhere if he doesn't respond soon, I have every other part of the rig on it's way right now 
Well, if he's fluctuation between winter and summer, that sounds to me like he may not be the most adept overclocker..
And since he's overclocked on Stock cooling, the wear and tear on the Chip is also generally higher, because (more heat stress)..
Regardless, there shouldn't be that large a fluctuation summer/winter fluctuation unless he lives in Africa and doesn't have access to air-conditioning..
Normally, I'd ask you to ask him what voltage he used to get 4.6, but given the information thus far, he probably doesn't know how to read the right voltage and if you taught him how, you'd come off as a bit condescending.. hahahahaha...
I'd say buy a new chip..