I was wondering how complicated it would be and what all it would entail to swap the 3.0l in my 1998 4x4 automatic trans ranger to a 4.0l engine?

 

Basically the same as swapping in a V8

You will need the 4.0l engine with its 4WD automatic transmission, and 4.0l computer with its engine wiring harness
1996 to 1998 4.0l OHV with trans and computer would be easiest , but only 20HP more, 140HP to160HP
2001-2011 4.0l SOHC with transmission would also need PATS key setup and transceiver added to ignition key cylinder, but 65HP more, 140HP to 205HP

2WD and 4WD transmissions are specific and no conversion is possible
Transmissions are also engine specific, so 3.0l trans can't not fit on 4.0l engine, or visa versa

V8 swap is the same, engine/trans, computer and wiring

Because Rangers came with 3.0l or 4.0l doesn't make the swap any easier, in fact it makes it less appealing because you can just go out and buy a 4.0l Factory Ranger probably for less than a swap would cost if you factor in your time
So you would find much info on doing this swap

V8 swap has more appeal since Ford never made V8 Rangers, well not for sale,lol
Lots of info on Ranger V8 swaps is out there