EPA numbers:
V6 2x4: 16 city, 21 highway
V6 4x4: 16 city, 20 highway
V8 2x4: 15 city, 19 highway
V8 4x4: 14 city, 17 highway
So if you are comparing apples to apples, it is a 2 mpg increase with the V6 (at best).
http://www.fueleconomy.gov/Feg/bymodel/2008_Toyota_4Runner.shtml