I'd check for a vacuum leak near number 2. Either a vacuum plug in on the manifold, loose manifold bolts or bad gasket.
Remotely possible an injector issue.
To much air or too little fuel can increase cylinder temperatures drastically. The 4.0 runs pretty close to the danger point, air fuel mixture, anyway. It doesn't take much to lean one past the safe point.
Pull the plug, get a good flashlight and look at the piston top. Most times it has a carbon coating, hot spots are white to mat grey or white to reddish. Really bad is a burned pit in the top of the piston, right under the spark plug hole.