Not many. CalTech is the one that comes to mind. [UPDATED: I was going from memory on this response, which led to some wrong info. I’m not seeing where any T30 school requires as much as four years of social studies. The highest I can find is Stanford, whose CDS says “3+” (recommended, not required). As @skieurope notes, a school would not reject applicants for having “only” 3.5 years of high school social studies.]