Regarding design specs, there are innumerable open designs available, including hawkboard and beagle board. Designing the schematic is almost trivial. Desigining the layout is only a little more laborious. Fabricating a four layer PCB is not difficult either but is an economy problem for small volumes < 500. Also a 4 layer board would be relatively big. Inorder to make it compact, the pcb would have to be 6 layer probably with bilind/buried vias. This makes the pcb more expensive, ie volumes of < 1000 per lot will be expensive (i have been getting mails from chinses vendors for volumes as low as 10 though). However no black art involved in any of the above, except the costs.
IIT Kanpur has facility to fabricate 8 layer PCB and I think I read somewhere that the PCB for this was fabricated at IITK