4 Vertical Prong Plug Adapter
This is an obsolete socket type called a parallel and tandem socket, and is designed to accept either standard parallel plugs without a ground or older tandem style plugs (again without a ground): 



None of the connections is a ground - it consists of 2 pairs of hot and neutrals, one oriented vertically, the other horizontally. If there is a ground wire to the box (which I doubt), this can be replaced with a standard 3 prong outlet. If not, you'd have to find a way to get a ground into the box.