mpg difference between 4x4 in 2wd vs 4hi?

let me start by saying im not asking the difference between a 2wd only truck vs a 4x4 truck. i know 4x4 has added weight etc..that hurt mpg. im asking if you take the same  truck that gets say 20 mpg highway in 2wd mode. about what would you say its mpg loss would be doing the same highway run but in 4hi. i know itll get less because now the engine is moving an extra driveshaft, axle, tires etc... but its not moving anymore weight and aerodynamics will be the same aswell. because that weight is still present in 2wd. im asking this question simply because i am curious what will happen, im not concerned about mpg lose in a truck. i understand its a truck and not a prius. i just dont know if its a small 10% reduction or as much as a 50% or more reduction. thanks