Maybe I'm missing the point but I thought the point in getting the Ranger was for fuel economy? The 4.0 is going to get very comparable gas mileage to what your 4.7 is getting. I have friends with 5.4 F150s and they are getting better gas mileage driving harder than I do. I keep up on my maintenance, drive the speed limit and I average 17 mpg doing 70-80% highway driving. All time best: 20.2mpg.
I know there are people on the forum getting gas mileage in the 20s with 2.9, but they are likely very well maintained 2.9s, which it can't be guaranteed you'll get for $1000. As for the 3.0, best I've got out of it is 23 mpg doing 90% highway, 10% city driving. If you look at the 4cyl though, there's plenty of people on the forum and elsewhere I'm sure getting mid to high 20s.
Fuel economy.gov (new estimates, 5 speed manual 2wd):
88 Ranger 2.9L: 17 city 22 highway
96 Ranger 2.3L: 19 city 24 highway
96 Ranger 3.0L: 17 city 23 highway
96 Ranger 4.0L: 16 city 21 highway
06 Ram 4.7L 6MT: 14 city 19 highway
Just saying a 5mpg increase is more than a 2-3 mpg increase.