Ground fill is counterproductive on the signal layer. If you accidentally get the return path on layer1 or layer4 instead of the designated layer2 or layer3, y

Ground fill is counterproductive on the signal layer. If you accidentally get the return path on layer1 or layer4 instead of the designated layer2 or layer3, you've created noise. Power+Signal / GND / GND / Power+Signal is about consistency and braindead-easy tracking of return paths. The return path for layer1 is always layer2. The return path of layer4 is always layer3. Keeping track of both the forward signal (or power line) and the reversed return current (which was electrically induced onto the nearest reference plane) stops working if suddenly you have random reference ground-fill planes on the layer1 or layer4. DO NOT put GND on layer1 or layer4 if you're doing this methodology. --------------- Beginners likely aren't working with a hot enough circuit where thermal dissipation is an issue. If you do have thermal dissipation then I guess thermal ground on layer1 and layer4 ties with thermal vias will be needed. In practice, the thermal resistance across the PCB cross section is better than beginners expect anyway. Thermal conductivity is just one attribute, the other attributes of heat movement are distance and cross sectional area. So the shape favors you up and down the PCB. Yes the fiberglass has worse thermal conductivity but you win on shape.