You should be good with installing a 130 amplifier alternator on your 91 4.0. There should be 2 plugs on there. One with 2 or 3 wires and one with a single wire..and the other wire that bolts on there from the battery. I can't say for sure how the ohv and sohc engines are. Make sure its for the ohv engine.
I looked after some pictures to verify this, go ahead and put one on there.
Around 27 years ago, i installed a 130 amp from a 97 Explorer i bought used as i did a very powerful audio setup in my 94 3.0 5spd ranger with no modifications. The plugs fit right in. I started upgrading all power wire and ground wires with all 4 gauge inside engine compartment Battery to alternator,. battery to radiator support, engine to frame, engine block to firewall..i did other 4 gauge wire upgrades in other areas. That alternator lasted 10 years..without any issues. In 2010 I went to the yard again and found a used oem one from a 99 Explorer and had it tested a couple of times at the auto store and it tested very strong. When my 94 3.0 ranger was totaled last summer it still had that same alternator in there.
Was is your battery size in your 91? If battery is two small with stock wiring it may not be able to keep that 130 working like it should..fro. my experience I've seen, can't say about others. If you buy a new or rebuilt one..have it tested prior to installing it. Here's a couple of pictures from my old ranger with 130 and maybe see the wiring upgrades.
I was also using a very large motorcraft battery.
