That #4 plug that's not getting spark is in a 1988 dodge Dakota 4x4 with a 3.9 litre v6

Since it's a 88 year model it doesn't have a separate coil over plug design as most cars today does. It using the conventional coil, distributor, cap, button, and wire ignition system. The first place to start is to remove the distributor cap. After you get it off turn it upside down and look inside it. You will see small metallic contacts inside I. Each contact represents and cylinder number. You can actually figure out which contact is #4 by tracing the plug wire from the plug back to the cap and see which contact on the top of the cap it is made a mark straight down to the bottom of the cap and straight back up inside the cap. See if the contact looks a lot different from the other contacts inside it. If not, check the rotor button. But since it's just one cylinder it's more than likely gonna be either, the dist cap, plug wire, or the spark plug itself. Of course there are other things it can be but one of them is most likely the culprit. To be perfectly honest with you. It's probably best to go to the parts store and get a set of good spark plug wires, set of spark plugs (Champion spark plugs I'll explain why shortly), dist cap, rotor button, pick up plate, and a small tube of die electric grease. The reason for using Champion plugs is because this is the exact spark plug that was engineered to go with this engine. If I'm not mistaken the plug number should be Champion RC12LYC. On the under side of the hood or on the radiator support there is a emissions, air conditioner, and displacement sticker. Look on it to see the exact plug if your not sure. Unless the truck has been wrecked and had those parts replaced. I worked in the R and D department for Diamlier Chrysler for several years. Hope it helps.