Life has slowed down enough for me to have a bit of time to spend with my insanity.
As I've mentioned before, my test jig can't drive enough pins to test either the Instruction Decoder or the Arithmetic Logic Unit separately, so these will have to be assembled and tested at the same time. I'd started a basic layout of the Instruction Decoder but hadn't gotten too far. This evening I made some progress.
All the components have been placed, though I'm not happy with the layout in some areas. A lot of the routing has been done, and what's mainly left is interconnecting the large functional blocks and to the off-board connectors.
For those who like numbers, I have 622 airwires remaining, 322 of which are missing vias from component pads to the ground plane on layer 2. Another 109 are to VDD, though since I don't have a dedicated power plane these will have to be routed just like the remaining 191 signals. I also need to add some bypass capacitors, as the original i4004 schematic has none; this will be especially important given the absence of a power plane.