Check your manual..(4 wheel drive for dummies)
Which one? most are not much more than removing a bolt or two. All but one will slide off without much effort. Let me know if you need more help.
Shift the 4 wheel drive lever into 4 wheel drive.
I don't know if a pathfinder is the same, but in my jeep the transmission has to be in neutral before it will go into, or out of, 4 low
Could be the 4x4 Control Module.