It's the same as virtually any current model 172. Last time i tried to put 4 people in one we were over MTOW before more than reserve fuel was added. And they weren't large people. We went in a 182.
Sure it's legal to register it, but it'd have to be illegal to fly it, surely that warrants investigation. Same as any late model 172 that 4 average sized people step out of.....I would wanna see a W+B for that effort.