During our conversations another option for MOSFET choices came to my attention. Rather than searching for one of the scarce few 4-terminal discrete FETs, it's possible to use two devices back-to-back to work around the problems created by the body diode created when the FET's substrate is connected to its source:
There is a drawback to this approach: the gate capacitance is doubled, since the two gates are in parallel. This will result in higher power consumption and possibly slower operation, especially since the 4004 uses resistive pull-ups rather than push-pull circuits in most cases, though there are push-pull drivers in cases where there is high capacitive loading (like in the DRAM arrays) or where timing is critical (like in the ALU).
Another drawback would appear to be increased parts counts. However, dual-FET parts are pretty common in the SMD world. Consider these two examples:
|BSS84W -- Single P-Channel MOSFET|
|BSS84DW -- Dual P-Channel MOSFET|
I think I'm going to add a few of these to my next parts order (along with prototyping adapters) so I can experiment with them. I want to see how they behave in a DRAM cell. That's the best part of hobbies -- there's always something to learn!