There is a fundamental problem with a 4.0L in a 4x4 ranger. It's a short stroke motor.
A short stroke motor belongs in a high rpm 2wd application. Not a low rpm 4x4 one. These things ought to have a slightly longer stroke than bore size.
And really.. nitrous can get expensive if your 16yrs old.. or constantly act like you are.
*IF* I were going to make power with my 06 FX4? I'd look into a single small turbo with an air to water intercooler. I'd max around 8-9psi at about 2800rpms, have the supporting fuel system to support it, and just not worry about the top end power.
*IF* I were to spec a totally new engine. I'd go with a 5.4L with the two rear cylinders chopped off.