I have successfully used two layer boards for some designs, but there is no question that 4 layer boards can be made to work much better for critical designs. Many designs are critical. The typical 4-layer stack-up will have a ground plane layer as layer 2, perhaps 7 mils from the top outer (signal) layer. This close placement helps to reduce inductance in high frequency signal traces.on the top signal layer. For symmetry, layer 3 would likewise be 7 mils from layer 4. If you want to have a lot of high frequency signals on the top and bottom of the board, you might want to have a 6 layer board with ground planes at layers 2 and 5 in order to help minimize inductance at both top and bottom signal layers.