When three or more layers are used, it is essential that one plane is used as a ground plane. It is also recommended to use one layer as a power plane if four or more layers are used. These two planes should then be placed next to each other in the middle of the board, to reduce power supply impedance and loop area. It is not a good idea to place the power and ground planes as the outer layers to act as shields. It does not work as intended, as high currents are running in the ground plane. A shield layer would have to be a second pair of ground layers.