Yes, I would say 30k-40k, 4.0l is only 9.0:1 compression ratio so doesn't need much.
Not sure why they would recommend that big of a gap on a 4.0l OHV.
I know sellers of Cold Air Intakes(CAI) recommend changing thermostats to 160degF, CAI's don't really do anything on fuel injected engines but the lower operating temp does, which is why they add that part, lol.
Yes, I would think .065 gap would cause issues on the stock 4.0l OHV
When super or turbo charging an engine higher voltage is better, or if engine has a higher compression ratio to start with.
Stock coil, wires and spark plugs in good working order will give maximum performance from the engine.
Nothing can be "upgraded" for better performance, spark system simply doesn't work that way.
If only 20k volts is needed to "jump the gap", at idle, then that is all that is used, even if it is a 60k volt coil.
If 40k is needed to "jump the gap", engine under load, and coil is only 30k then you can get misfires.
Spark plug gap can be changed for driver preference, as said earlier.
Screamin' Demon coils are fine as far as I know, but they don't increase performance over a working stock coil, unless engine was modified to need a higher voltage output to "jump the gap" under load.
Any spark that can ignite the air/fuel mix works.
The spark comes at approx. 30deg before top dead center, as the air/fuel ignites around the spark plug the pressure increases which snuffs out the spark, so minimum voltage spark, rest goes to ground.
The ignited air/fuel burning spreads and complete explosive combustion should happen at about 10deg After TDC, that gives the best leverage to push down on piston to give crank the most power.
So voltage needed from coil is based on the engine , higher voltage certainly won't hurt anything, and no it can't burn pistons, popular myth, lol.
But it also doesn't really do anything to have more voltage than engine requires at maximum load.
Weak spark, to low a voltage from coil or bad spark plug or wire, nothing manufacturer would install, weaker spark can't ignite richer fuel mix, so slow cold starts and misfires under load