The PD32 PCB routes in in FreeRouting as a 2-layer board, however, based on what I've read the NS32K chipset seems rather sensitive to signal quality.  A 2-layer board is less expensive, but if a 4-layer design is more likely to work, then I think that's the way to go.  

It seems there were some articles and/or letters in MicroCornicopia after the original PD32 article suggesting PCB quality caused a lot of problems to the initial roll out in 1986.  They also used an improved 4-layer board to improve signal quality, so I think it is prudent to do the same.  

The good news is the PCB easily routes as a 2-layer board so transitioning to a 4-layer PCB should be no problem.  Just add two internal copper layers, one for GND and the other for VCC.