San Francisco is legally a city, but I think of San Francisco in a town way.
San Francisco is legally a city, but I think of San Francisco as a town.
San Francisco is legally a city, but I think of San Francisco in a town way.
San Francisco is legally a city, but I think of San Francisco as a town.
The correct idiom is "think of A as B." The structure "in a [noun] way" is very colloquial, and not acceptable on the GMAT.