the NFB resistor is on the PCB and is a 10K (R7) resistor, along with a 680pf (C11) capacitor.
These values are optimum when the NFB wire is connected to the 16 ohm tap,
as is the normal practice.
If you are taking the NFB connection DIRECTLY from the 4 ohm connection,
you need to half the value of the resistor to 5.1K and double the capacitor value.
From your note, you have two MK3 amps and one of them works fine, so you should be able to compare the two directly to find any problems. It seems your B+ voltage is a little high, and if you are still using an original quad cap you may be stressing or overloading it, as they are rated at 525v.
That would definitely cause a hum problem. But you are saying you only hear this noise when connected to the 4 ohm tap, and only on the one amp?