Another thing I thought of (but you may already have checked) is the control signal wire that connects to the starter solenoid.
This wire is notorious for breaking on the 4Y engines (among a list of other wires that break).
So, if you are hunting a starter problem on a 4Y in a 7F, make absolutely sure the black control wire at the solenoid connector is not broken. Physically pull on the wire, even if it looks OK.
I have seen them break but still be held in place by the wire insulation a couple of times.