If you want to make a commercial product, bear in mind that a THOUSAND VIAS will still cost less than a four layer board (which is basically two two-layer boards glued together). The only reason I can ever think of going into multi-layers is either Screening or Power Dissipation purposes. With good CAD software (and a good PCB manufacturer) your track separation can be as little as 0.15mm and there should be no need for four layers.

(For those who don't know, a VIA is a tiny plated hole connecting two layers of a board.)