I pulled the ignition coils out of my 4.0 and each one had a crack in the plastic around the coil. The boots all appeared to be in good condition.

Is this a purely cosmetic problem or would this impede the operation?

Generally speaking the car runs great.

 

I would consider that as a break in the insulation that isn't necessarily going to cause a problem unless something conductive gets down the hole, i.e. water or oil. If one starts shorting to ground, you will know it.

Normally I would take this opportunity to shamelessly offer to sell you good used ones, but I've seen new coils for sale around the ether for like $70.00 each. I have one new one if that helps, but it sounds like you're doing fine as is. PM me otherwise.