Edit: Nevermind - ran a jumper between the positive battery terminal, and the ignition wire going to the solenoid - truck started. So, it's the ignition wire, not the solenoid.

I may need to replace my starter solenoid. This is for a 95, 4 cylinder, 5 speed Ranger. The exact replacement, would be this Motocraft one with 3 posts:

https://www.rockauto.com/en/moreinfo.php?pk=386544&cc=1138025&jsn=499

However if I end up needing a replacement - I'd love to grab a part locally. The local ones all have 4 posts, like this:

https://www.autozone.com/batteries-s...84961_213556_0

Is that...OK, and one can just ignore the 4th post?

 

The older 4 post models should have labels on the smaller posts, "S" and "I", the one pictured doesn't

"S" post is for starter activation, "I" post for Ignition Coil power while cranking engine

Back when spark systems used a single coil, the coil would run too hot when it had the full 14volts from alternator, so a Resistor was added to the coil's "+" terminal to lower the voltage to about 8volts engine running.
But when you tried to start cold engine, battery voltage drops to 10v when cranking, the resistor would cause too low a voltage, 4volts, at the coil so very very weak spark
So the "I" post was added, it connected to Coil's "+" terminal, when starter relay(solenoid) is activated the "I" post was also powered, 10volts, so coil could do hotter spark for cold starts, after startup the "I" post was disconnected and coil gets it 8volts

In later years car makers just ran separate wire for coil voltage from ignition key with diode, it bypassed resistor for startup voltage