Still new to this forum, and new to being a Toyota 4Runner owner... but have plenty of experience towing with a smaller SUV.
My former 2005 Nissan was rated at 5000 lbs towing, 265 Hp, 284 ft-lbs of torque. The trailer weighs 3993 lbs empty, but with the options. Pushing 5K when fully loaded. It had no problem pulling the weight, no signs of sway, and plenty of reserve power. But I did turn O/D off to keep the RPMS in the power band. There was a lot of armchair quarterbacking on the Nissan forums regarding whether it could tow a trailer like this.
I towed this camper many times, and put 90K on the odometer before buying a larger tow vehicle as we wanted to haul more stuff (hence my handle).
I used a Prodigy brake controller, and a properly configured Equalizer hitch. Notice that my vehicle was level, as was the trailer.
The 4Runner has the advantage of a longer wheelbase, but the HP, torque, and tow rating are very similar to my previous vehicle.
I have no doubt the 4Runner will tow AT LEAST as well as the Xterra. I will be using it in the future to tow this same camper. I'll be buying the brake controller this weekend.
As for the "80% Rule", there isn't one. It's netlore. You won't find it in your owners manual. The safety margin is already in the manufacturer's figures. This has been discussed ad nauseamon the motorhome forums. If 80% is your fuzzy feeling figure, more power to you.
And those who say a gasoline engine isn't good for towing, ugh. My F150 is rated at 11,100 lbs of towing, and 1810 lbs of payload capacity. All with a 5.4L V8 gasser. That's a 1000 lb golf cart in the bed.
And when I had a 36' 10" Class A motorhome, towing a Jeep, it had a V10 gasser. Google F53 chassis to see the most popular RV chassis.
So just as before, I'll be towing near the rated capacity, with the addition of a brake controller and a weight distributing hitch. Expect to see pictures in the near future...