All the fuses get their input +12V on the Left side terminal (closest to the door).
Some fuses (6—10) have +12V there all the time.
Others get +12V under certain conditions (e.g., fuses 1—3 when the Key is On).

Fuse 4 gets its +12V from the Fuel relay, when it gets energized by the FI ECU.
But the FI ECU must "see" timing pulses from the Ignition Controller first.
Those ICU "timing pulses" originate with the Engine Speed (RPM) sensor
(aka Crank Position sensor) when the engine is turning.

So there WILL NOT be any +12V at fuse 4 until the engine starts cranking, and
the RPM sensor sends timing pulses to the Ignition CU, and
the ICU relays those timing pulsed to the FI ECU, and
the FI ECU energizes the Fuel relay to power the pumps, injectors, and the rest of the system.

If fuse 4 never gets powered, the problem could be:
1) RPM sensor (common with age)
2) An ICU problem (extremely rare)
3) An FI problem (i.e., an aged Fuel relay)
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.