In SX Microcontrollers, SX/B Compiler and SX-Key Tool, dpaton wrote:

I do 2, 4, and 6 layer boards on a fairly regular basis.

Given the choice, I'll always pick a 4 layer board, because it allows me to put power planes on the inside layers, or routing on the inside and ground layers on the top and bottom for critical systems, or 4 layers of routing with integrated planes for dense boards, or...

The main advantage over 2 layer designs is the possibility of increased density and better plane layout, which are often used together to remediate the noise that can crop up when things are packed together very tightly on a board.

The last option I can think of right now is for very high current circuits. I have one spot on a board that will be measuring current in excess of 100A, and I have the copper pours for the connectors and the measurement chip on all 4 layers, with no soldermask on the top and bottom so they'll be coated with solder during the solderwave process and have a little more metal to spread the current out in. Every little bit helps when you're working with BIG power.

Silkscreening is really really really nice to have, and I consider it only slightly less of a requirement than the ubiquitous green LPI (Liquid Photo-Imagable) soldermask. I'm big into labels, legends and notes. Permenant things like copyrights go into the copper layers (one mark on each layer, no grinding one off), along with things like part #s, logos, and things like that (usually on the top or bottom copper layers only). Connector pinouts, component outlines, serial number plate areas, and other things all go on the silk layer. 

My $0.02

-dave

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