Which figure always has exactly four lines of reflection that map the figure onto itself

A square has exactly four lines of reflection. The four lines are: the two diagonals of the square, and the two lines that are parallel to the two sides of the square and cut the square into two equal halves. No matter how big the square is, along these four lines, you can always fold the square so that the two parts match up with each other. Other lines do not work.