A four-Layer PCB has four copper layers. The top and the bottom of a four-Layer PCB are usually used as routing layers. The remaining two internal layers of the four-Layer PCB are used for power and the ground. Unlike the 2-layer PCB, the four-Layer PCB is a is a multi-layer stack-up design. It is important to note that a professional four-Layer PCB stack-up design will produce much less radiation (around 15dB less) than a two-layer PCB design.
Between the four-Layer PCB Copper layers is the Core and the Prep reg. During manufacture, all these elements are brought together (sandwiched) by a laminate under high heat and pressure to ensure that the whole stack-up is held together in place. The four-Layer PCB can contain through vias, blind vias and buried vias. For a four-Layer PCB, buried via can only be between the second and the third layer and the blind vias can only be between top (first) and second layer or between bottom (forth) and the third layer.
Getting your four-Layer PCB manufactured is not expensive if you use a company like wellpcb. To keep this cost as low as possible, you are advised to use buried and blind vias only where very necessary and if it is the only way out.