The three post configuration is what would be considered the original setup, and should be all you need. The 4 post that is on there is likely from an automotive application: That 4th post takes the 12v+ from the large battery cable and sends it to your ignition coil, to alleviate the problem of low voltage through the ignition switch while hitting the starter. On the 35"s it isn"t really an issue- especially with 12V, since they crank over so easily (therefore not dropping the voltage enough to affect the ign. coil), so the 3 post solenoid you purchased will work like new for you. All you need to do is determine which wire comes from your starter button, and put that wire on the little post of your solenoid. The wire that was originally on that 4th post will not be needed anymore- you can just tape it off or remove it- it should lead to the + side of your ignition coil. One of the two big posts goes directly to the starter, the other directly to the + battery side.
Best of luck and congrats on your "new" Fergie
P.S. If you ever decide to paint this tractor, the original colors should have been dark green on the cast iron, and light gray sheet metal I believe, if you like keeping stuff original.