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You didn't say what year your explorer was, but I believe the spark plug gaps on the OHV 4.0 would be between .052-.056 . As for the single vs. double platinum goes I've heard of people running the double plats with good success and no problems. The double plats would last a little bit longer but that would be about it in my opinion. If you've got single plats you can go ahead and use them without problems. As for the coil packs, there are 3 packs combined into one with 2 plugs per pack.
There should be numbers on the coil pack that identify which 2 cylinders that individual coil is suppose to fire. If I remember the coil numbering goes like this.
3 6
2 5
1 4
Front of Engine
So in this case it would be 1-4, 2-5, 3-6 would be the 3 ignition coils and what coil controls what. The 142536 is also the firing order. The cylinders are also numbered in the same manner as the coil firing order above, so hopefully that helps.