I think the 4Runner is a light-duty or maybe a medium-duty tow vehicle. It's just not big enough and heavy enough to tow big loads, regardless of whether or not you have a V8 or a V6. Personally, I think 5000 lbs would be about the heaviest load that ANY 4Runner should tow. I know someone who tried to tow a horse trailer (2 horses) with a 4Runner and had nothing but problems.
Bottom line-- if you're towing heavy loads, do it with a different vehicle, IMO.
I have found that the effectiveness of the 4Runner as a tow vehicle is directly related to what it is towing and what towing equipment is being used.
The same can be said for larger pickup trucks that tow as well.
Forgive the pun but towing horses is a COMPLETELY different beast when it comes to towing. The constantly shifting loads involved with pulling a horse trailer can pose a challenge for any tow vehicle (including a 4Runner or a HD Dually pickup). That is FAR DIFFERENT from more STATIC loads that do not move around nearly as much.
Where many people go wrong when it comes to towing is exceeding the towing capacity of their tow vehicle. Usually they compound that mistake by not using weight distributing hitches & receivers and sway control. They also do not BALANCE their load PROPERLY and get themselves into even more TROUBLE.
Personally, I tow an 20ft, enclosed car trailer (weighing about 5,500 lbs) with my V8 4Runner. I have a weight distributing hitch & receiver and dual cam sway control. I have used this setup towing in 30-40+ mph cross, head and tail winds and been in total control at all times. Funny how the same could not be said for the Heavy Duty Dually pickups pulling trailers that were swaying and porpoising all over the roads.
BTW, the orignal poster should realize that almost all on-line towing forums shun all tow vehicles unless they are BIG, HD Pickup trucks. They particularily dislike Toyota 4Runners for some reason.
Bottom line is if you stay with the specs and use the proper equipment the 4Runner is a capable tow vehicle. If you overload it or do not use the proper equipment you are asking for a disaster. Also, if you have never towed before I STRONGLY SUGGEST that you seek out someone who has towing experience to TEACH you how to TOW PROPERLY. There are many, many, things one needs to learn to tow properly & safely. Learning those things "on the job" can be dangerous and expensive. I was fortunate to have a friend with a LOT of towing experience (and a CDL license). The information he taught me was very, very, valuable and made me a MUCH Safer driver when it comes to towing.
FYI, a couple of weeks ago I pulled not ONE but TWO Ford F250 Super Duty Duallies (one at a time) out of the mud they got stuck in at a car show. The owners looked like they thought I was crazy when I offered to pulled them out. Funny how that look changed to a look of utter disbelief when the 4Runner pulled them out without even breaking a sweat. I am still looking for someone who took photos of me helping them as I was too busy to stop and take photos myself.