4 layer PCB simply means you have 4 layers to rout electrical signals. that is 1.Top layer 2. inner layer1 3. innerlayer2 4.Bottom layer.
here TOP and bottom layers are the outer layers where we are placing componets and do some routing
inner1 and inner2 are normally used as power planes ( VCC ,GND etc), can can be used for signal routing also. It depends on the complexity of your board and nature of signals (RF,analog etc)