Original technical explanation
A 2 layer PCB routes copper on the top and bottom sides of the board. A 4 layer PCB adds two internal layers, often used as power and ground planes. Those planes can improve routing organization and electrical behavior, but they also add manufacturing complexity and design priority.
How to choose
- Use 2 layer PCB when the design is simple, straightforward and not too dense.
- Use 4 layer PCB when the board needs better power distribution, ground reference or compact routing.
- Review stackup early if impedance or strict thickness targets are involved.