Starter will not disengage?

I have an 84 bronco II with a 2.8 L V6. At first the starter was making a clicking noise and shortly after nothing at all so I replaced solenoid hoping to solve problem with cheap fix. NO LUCK, go figure. So I bought a remanufactured starter and replaced. Now the starter will not disengage even after the key is turned to the off position. The new solenoid has 4 posts coming off of it, one of which I have nothing to connect to, because the original solenoid only had 3 posts. So do I need to connect a ground or some sort of jumper wire to new solenoid and by the way I can't find the old solenoid, So putting it back in is not an option.