I took a look at the Instruction Decoder board this evening and discovered there were a handful of parts that not only hadn't been routed at all yet, they weren't even in a near-final placement. I really thought I'd gotten farther on this than I had.
It wouldn't show up on a screen shot so I'm not going to bother, but I juggled most of them into a reasonable layout and routed within the groups. I also took a look at what's left unconnected and, like the ALU board, switched from a 4-layer to a 6-layer stackup using the same layer assignments as the ALU.
Ratsnest now reports 16 VDD, 32 GND, and 85 other airwires. That's after I'd already routed a bunch of unconnected GND pads. Again, I'm a bit surprised these numbers, especially the number of GND airwires.
There's still some possibility that as I get close to the final routing I'll decide to go back to a 4-layer board. Since all the components except the connectors are surface-mount, I've routed as much as practical on the top layer to minimize the number of vias (and thus minimize the board area they occupy). Looking at only the bottom and inner layer there really isn't that much there. If it's practical to route the remainder using only these two, and if the power distribution will also fit, I might dispense with the extra two layers. Or maybe not.