I'm designing a circuit that consists of the following: AT Mega 128L 2 to 6 CPLDs SD Card Interface 100 to 200 MOSFETs Power circuitry I should add that I already have a 2-layer prototype of the board and it works well. However, the prototype does not have the MOSFETs and the SD Card Socket. The prototype is also on a smaller scale - it only has two CPLDs. The circuit is actually a automobile wiring harness checker. The harness can be upto 300 wires and is connected to the output of the CPLDs. The MOSFETs act as buffers for output of the CPLDs. I'm using a very small package to conserve space and the intent is to mount them on both sides of the board. In the current layout, 90% of the routing is on the top layer and the bottom layer is ground. However, as it's quite difficult to route everything on a single layer some tracks inevitable are routed through the bottom layer - which leads to a not-as-effective ground plane. With a large circuit, consisting of say 6 CPLDs, this will become more of an issue - and even more so because I'd like mount the MOSFETs on both sides of the board. So my question is, should I go for a 4-layer board which will allow me to have a dedicated power layer (3.3V) and a dedicated ground layer and two complete layers through which I can route the signals. My other question is - as I intend to include the voltage regulators on the board, do I simply source the power to the power layer using a via which connects to the output of the regulator? Or should I have multiple vias? My frequency of operation isn't very high (max. 4MHz) but I can even bring it down to 62.5kHz. Speed isn't important as the amount of data isn't large. However, signal integrity is very important.