Just curious...could one take out the 3.3 V6 and replace it with the 2.4 I4 and still be compatible with 4WD? I know Nissan used to make the Frontier in a 4 cyl. 4WD, was that the same transfer case, transmission, etc. that we have? Was it the same engine that the current 4 cyl Xterras have? I believe it came only in 5-speed manual, so would that make a difference? For instance, could you only swap it with a 5-speed?

Are you out of your mind? Why would anyone want to go from a 6 to a 4? To save gas? The cost of the swap would NEVER be recouped in the form of fuel savings.
You need an entirely different computer and wiring harnesses from a 4-cylinder Xterra, plus the transmission and transfer case from a 4-cylinder 4WD Frontier since the V6 transmission won't mate to the 4-cylinder engine, nor are the transfer cases interchangeable between 4 and 6 cylinder models. Also, the V6 4WD's R200A front differential won't clear the 4-cylinder's oil pan, the 4-cylinder 4WD Frontier had the R180 diff.

Thanks for answering my question. I really only wanted to know firstly because it would give me a better understanding of my current engine and the nature of how and if it can fit with certain other Nissan parts, thereby giving me a better understanding of my truck in general.
The other reason I ask is that in the case of completely blowing my current 3.3 V6 (in a few hundred thousand miles) and being forced into purchasing a new engine, would it be physically feasible, and if so, an economically sound choice to install the arguably cheaper, smaller 2.4 I4 engine currently used in some Frontiers and Xterra XEs.
Not out of my mind....just thinking out loud. Thanks again.

While we're on the subject, what's the possibility of swapping the new 4.0 from the '05 into say, an '01?

Cheaper to buy an 05.

You would need the engine, probably everything electrical, fuel and emmissions related. New transmission, new transfer case. New frame work or brackets to mount it. Possibly a new front end if diff-oilpan clearance is an issue. The list could get very large.