You need 4 to be able to isolate the current flow path.
In one direction, the current flows from upper left to motor + to motor - to gnd at lower right.
In the other direction, the current flows from upper right to motor - to motor + to gnd at lower left.
If you're only switching direction occasionally, then you could get by with a DPDT switch and one signal to enable/disable the power transistor.
Will post a drawing in a minute ...
