Most of the circuitry in the 4004 CPU and other MCS-4 family chips uses a MOSFET to actively pull a signal toward Vss, and a "load" to passively pull it back toward Vdd. In the real 4004 CPU, a "load" is a MOSFET with its gate connected to its drain, but designed so it works sort-of like a resistor. In my re-creation, a "load" is a real resistor.
When I did my initial breadboard tests of bits of circuitry, I used 10K ohm resistors for loads. I chose this value simply because I happened to have a bag of a hundred of 10K ohm axial resistors left over from another project. Given the low input capacitance of the two MOSFET types I am using (9.5pF for the Fairchild FDV301N, and 1.5pF for the NXP BSS83), it seemed a reasonable choice.
When I began assembling my boards, I worried that 10K loads wouldn't give good rise times. Back when TTL logic was standard it was common to use a 4.7K pull-up resistor on open collector outputs, so I used these instead. I also didn't understand how "bootstrap loads" worked, and thinking this was a rise time issue I used 2.2K resistors for these. This seemed to work pretty well on the Instruction Pointer board, but I had no engineering data to support these choices.