Four-Layer PCB - How To Make?
A four-Layer PCB has four copper layers. The top and the bottom of a four-Layer PCB are usually used as routing layers. The remaining two internal layers of the four-Layer PCB are used for power and the ground. Unlike the 2-layer PCB, the four-Layer PCB is a is a multi-layer stack-up design. It is important to note that a professional four-Layer PCB stack-up design will produce much less radiation (around 15dB less) than a two-layer PCB design.
Between the four-Layer, PCB Copper layers are the Core and the Prepreg. During manufacture, all these elements are brought together (sandwiched) by a laminate under high heat and pressure to ensure that the whole stack-up is held together in place. The four-Layer PCB can contain through vias, blind vias and buried vias. For a four-Layer PCB, buried via can only be between the second and the third layer and the blind vias can only be between the top (first) and the second layer or between the bottom (forth) and the third layer.
The typical stack-up for a 4 layer board would be power & ground for the inner 2 layers, and then the signals on the outer 2 layers. Normally one would route the 2 signals layers in the perpendicular fashion. It's not as critical if the layers are separated by power and ground, but if you have signals on adjacent layers it becomes more important so that you minimize crosstalk.
As for numbering one usually goes from 1 to n starting at the top going down to the bottom. This is the only convention, you can do whatever you like, as you'll provide the stack-up info when you send the files for production.
Getting your four-Layer PCB manufactured is not expensive if you use a company like WellPCB. To keep this cost as low as possible, you are advised to use buried and blind vias only were very necessary and if it is the only way out.