If you have the 4.0 already and it is in good shape, i say def swap it in. If you are going to have to buy it, I say forget it and go for a v8 swap which I understand is cheaper and easier than the 4.0l swap. We all love our 4bangers but I can't imagine any of us here TRULY passing up the opportunity for proper power. Have any of us really NEVER wondered why Jeep offered a 2.5l when the 4.0 was so damn good (easy now guys, I'm not bashing).
AS for gas consumption....that's debatable. They are quite close from the factory, if not the same. Further, with add ons, tires, armor etc, the 4banger is going to consume more pushing all that weight around.
Unless you're going to pull out the 4banger and strip it down to the block and rebuilt it into a monster, turbo or supercharged beast, I say swap in that 4.0.