2-layer PCB is enough to route any circuitry, at least in theory. One layer for vertical tracks, the other for horizontal tracks, connect them with via’s.
If you need route high-speed signals on the PCB, such as RF circuitry, switched-mode power supplies, and long digital buses, 4-layers are required. They are widely applied in modern datasheets, high-performance semiconductors, switched-mode PSU, heat-rejecting devices.
- It can ensure the signal integrity, even though the return path for the signal currents is too long and the loop impedance is too high.
- The radiated EMI will be steady so as to pass CE/FCC.
- ensure that the device won’t emit too much radiation and would be robust against interference.
Price of a 4 layer PCB is roughly 3x-4x higher than that of a 2-layer one. However, most of the time 2 layer is not a viable option.