F4 circuit is an unswitched circuit, comes straight from battery, hence the reason if there was a short to ground in that circuit it would blow F4 fuse even with key off. You're going to need a multitester that will test both voltage and ohms (or continuity). Pull F4 fuse, test the two terminals in the fuse box receptacle with your volt meter to determine which terminal is the battery terminal. Once you determine that, the other receptacle will be the circuit that goes to the main controller (computer). Test that receptacle with ohm meter (or continuity tester) with one lead connected to receptacle and the other lead connected to ground. If it's blowing F4 you should see continuity to ground on your tester.
Once you've verified that, disconnect J1 connector from the main controller. Check receptacle at F4 again, see if you still have continuity to ground. If you do, you have a short circuit to ground in the harness that runs from fuse box to J1 at main controller. If you don't have continuity to ground, you need to test the connections at main controller.
Once power leaves F4 the circuit splits off into 3 wires, wire 1110, wire 1120, and wire 1130 and they go to pins 13, 28, and 33 at J1 connector to main controller. With J1 disconnected, test pins 13, 28, and 33 at J1 of main controller to see if one of those three show continuity to ground.