Was around a couple years ago with an old F150. Never got much of a response out of it as far as fuel economy goes. Trying to get some miles out of a Ranger now...
Not a whole lot done to it as far is fuel economy goes. Guess the underdrive crank pulley and synthetic fluids help some. Just finished converting the mechanical fan over to an electric taurus fan. Hopefully that'll help
Depends which 4 banger. The older ones were gutless, but the new 2.3L Duratecs are only rated for about 10HP less than my V6 but easily get 5+ MPG more. Not sure which year ranger started getting the Duratech.
As for the truck, I paid about $3500 for it a year or so ago. Moderate miles (around 100K) and good condition.
EDIT - Mid 2001 and up rangers could get the 2.3 Duratech which had 143HP (compared to my 160) BUT is rated for about 5mpg (mixed) over the 3.0.