Jumping Frogs

Four frogs sit on integral discrete lattice at points (0,0), (0,1), (1,0) and (1,1) and form a square of length 1. Each frog can make a mirror jump over any other frog. For example from (1,1) over (0,1) you to (-1, 1). Prove that the frogs cannot form a square with a length greater than 1.