The most important difference between PCB four-layer boards and double-sided boards is their number of layers and structural complexity. Specifically:

1.Number of layers:

Double sided board: there are only two layers of conductors, usually consisting of an upper layer and a lower layer, with an insulating layer (substrate) in the middle.

Four-layer board: has four layers of conductors, usually a top layer, a bottom layer, and two layers in the middle (usually the power and ground layers) with two insulating layers in the middle.

2. Design complexity:

Double-sided board**: for relatively simple circuit designs, suitable for fewer connections and signals.

Quad Panel**: can support more complex circuit designs, enables better signal integrity and power management, and is suitable for high-speed signal transmission and more component layouts.

3. EMI and Signal Integrity:

Four-layer boards can more effectively suppress EMI and improve signal integrity through internal layers (e.g., power and ground), whereas double-sided boards may be more susceptible to interference.

4. Applications:

Double-sided board: commonly used for simple electronics or low-density designs.

Quad Layer Board: Commonly used for products that require higher performance and more complex functionality, such as computers, communication devices and consumer electronics.

In summary, quad-layer boards offer higher performance and design flexibility compared to double-sided boards for more complex application scenarios.