Ehhhhh maybe to answer your question. This was my girlfriend's 4th gen a few years back:
I think our travel trailer weighed in around 5500lbs empty if I remember right. The 4runner is a 2003 (so doesn't have VVTI/more power like 05+) and we bought it was 215k on the clock, and it towed the trailer surprisingly well. We towed that trailer from the mountains in Colorado to where I went to school in Northern Utah, and then back to Colorado, and then from Colorado to Southern Utah where she now goes to school--so this was all towing across the Rockies and 4000-9000ft. Was it fast? No. It could hold about 70 on the interstate if it was flat, but much of a hill and it started working for it. If you want a 4runner, and you plan to tow around on little highways, I think you'd be fine if you're not worried about going up passes at 60mph. The 4.7 is a workhorse, and you can get them relatively cheap (like I said ours had 215k, and was all stock with no rust for $4700) and they run forever. The only issue we've had with the truck was the exhaust manifold leaking which is a common (non-critical) issue, and we're just now fixing that this week as the truck is approaching 270k and runs great as ever.
The original mods I did on the truck were specifically for towing, I upgraded to the sport calipers and EBC drilled and slotted rotors plus all Fox suspension with compression adjusters in the back and 2" lift Icon rear springs, that topped with dual braking axles on the trailer and an equalizer hitch made the 4runner sit about level and feel fine.
Now with all that said, I wouldn't push it much over that if you didn't have to.
This trailer is single axle (shittily) braked, and the trailer alone is 2500lbs and no equalizer. It towed my FJ40 plus a dirt bike and my 600cc sport bike from Utah to Colorado, but it wasn't happy (relatively speaking, it still towed it up really steep mountains surprisingly well). And we've tried towing my old racetruck (that the Tundra has in the top photo) and the 4Runner couldn't handle that at all--but that load was also 8000+lbs, and why we got the Tundra.
Conclusion, depends how fast you want to go and if that's the most you plan to haul, I personally wouldn't hesitate. Just have realistic expectations, it is a 5000lb truck stock and 15+ years old for the older ones.