Both double-sided PCB and 4-layer PCB are commonly printed circuit boards. When to choose a double-layer PCB and when to choose a 4-layer circuit board, this needs to be determined according to different application requirements, including: 1. Since 4-layer PCB has more routing than double-sided PCB, 4-layer PCB can have more functions in a smaller space. 2. If your project is more complex, a 4-layer PCB is required. 3. If your application requires good durability, please choose a 4-layer PCB. 4. If you consider a reasonable budget, it is best to choose a 2-layer PCB. 5. Due to the consideration of delivery time, the delivery time of 2-layer PCB is faster than 4-layer PCB. In short, you also need to decide whether to choose a double-sided PCB or a 4-layer PCB based on other more situations.