I added cruise control to a 2001 Tacoma using the Toyota kit. Toyota offered a dealer installed kit for them. I dunno, they might still offer them on cars now, too.
Anyway, I used part number PT474-35981. Was about $200 from a dealer parts department. I'm not sure if the kit would be the same for a 4Runner or not. I'm also not sure if the kit is even still available, I did it about 3 or 4 years ago now and ended up having to sell the truck not long after.
These are the instructions for it:
http://www.parksoffroad.com/tacomamods/miscinfo/tacocruisecontrol.pdf
It wasn't difficult per say, a couple of wires, some parts you remove and replace, etc. Took a few hours and honestly the hardest part was drilling the plastic trim on the steering wheel and that was just making sure it was clean and positioned right.