Background first. There is a big difference in film TTL and digital iTTL. Film TTL always fired a full-power flash, and then metered the flash as actually reflected from the films surface, literally, and when deemed sufficient exposure, quenched the flash OFF abruptly.
Digital cannot meter this reflection, and instead meters a weak preflash, and programs a lower power level in the flash (which is also just a short duration of full power flash, but programmed in advance).
SU-4 "Auto" mode was a film TTL method of doing multiple film TTL flashes. Film TTL could not meter individual flashes, but when this reflection from the film surface was deemed sufficient, all flashes were quenched. Film implementation was done in two ways:
The "three pin" "TTL Cables (SC-18 for example) had four wires in them, to each remote flash, and one pin was the Quench pin. The current SC-28 hot shoe extension cord still has a three-pin connector, which still works for manual flash, but this connector does not work for digital iTTL flash.
For film wireless TTL, the SU-4 was used. SU-4 is actually an optical slave accessory we can still buy:
Nikon SU-4 Wireless Remote Slave TTL Flash Controller 3070 B&H . The same function is built into some of the Nikon flash models, called SU-4 mode. In Auto mode, it can watch the master controlled TTL flash, and when it quenches, it can quench the flash on its shoe - echoing that TTL control (for film cameras).
But digital iTTL does not work that way today. We might argue that theoretically it might still do a similar function, but there is no control that way. Digital iTTL instead uses the Commander to individually meter each remote flashes preflash, and preprogram each one to the correct power value.
I would strongly suggest using SU-4 mode in its Manual mode, and set the correct power level in the remote flash (perhaps via metering it with hand held meter).
