Homebrew PCBs

I've done some home-brew 4-layer boards, but without plated-through 
holes.  It's tricky, you need to design for it, leaving plenty of room 
on the outside layers where you want to connect to an inner layer, so 
you can drill a hole big enough to stick your soldering iron down into. 
  It's not the most reliable results, either, but for some throw-away 
boards it's the best solution.

For professional boards, fabrication involves at least one drill step 
that drills all the way through all the layers, then a chemical bath 
removes the smeared epoxy from the drill bit, exposing the edges of the 
copper for the inner layers.  When you then electroplate it, it makes 
electrical connections between the layers.