Yes this has been done, a buddy of mine did it. You have to remove the V6 engine mounts and weld in the 4cyl., you will also need the transmission and t-case for the 4cyl, and you will need the complete wiring harness and ecu from the 4cyl rig as well(for ease of install).

The real question is why would you want to do the swap?

The only reason why my buddy did the swap was that he traded another buddy of ours his built '98 Runner for the X-cab w/bad V6 and an SA's 2wd rig with everything needed for off road(i.e., dual ARB's, gears, dual cases, etc.). He was tired of wheeling the Runner and wanted to build a truck.

You will be giving up power/torque/ performance of the 6 for the 4 cyl, even if you do a power build-up for the 4 cyl. Granted the 4 is easier to work on and gets a little better mpg's, but IMO not worth the sacrifice. I have 2 x-cabs one w/the 6 and one w.the 4cyl, and the 6 beats the 4 hands down, even w/the 6 weighing an easy 1000lbs more.

If your current engine is bad, you would be better off rebuilding it, since the costs of doing the swap will likely be more than rebuilding the 6, that is if you do the swap or rebuild yourself.