87 pathfinder trany swap

I have an 87 pathfinder SE with a 3 speed transmission and it's slipping between 2 and 3
I red about the 4 speed in a magazine a while back and I was wondering if I can put a 4 speed on my rig.
ID like the fuel economy to go up just a little. Any ideas or tricks you have I would love some ideas.  

What kind of question are you asking, about fuel economy?, Maybe But depends on what you have vg30e is only 150hp, but a torque monster for what it is, i have a 1993 pathfinder and i only get about 15mpgs(city) well after a few mods, 17. but they will take for ever to pay them selves off because that's not a huge difference and i cant keep my damn foot out of the throttle. 4 speed may help but never much, don't get me wrong i love my gas hog, but hybrids have the advantage the use a generator motor that propelles them from a stop, so that takes care of a lot of the fuel consumption, more gears better econ, true but only so far, that motor still has to whined up and get that rig going, so is it worth it economically, my opinion no, is it possible, yes Nissan is pretty good at making a lot of components interchangeable, trannys being one of them, another issue is if its a 4x4 you have to find one of these 4 speeds compatible with transfer case. at least that's my opinion, i am in no way a auto Einstein but that's where i would stand.