I'm currently towing a 4200-or-so pound TT with a 2010 4Runner SR5 with the 4.0 liter, V6. It handles the job very well; no problems at all. So, your lighter weight trailer would be fine.
The Highlander may be a different story. There are a few different varieties of Highlanders out there, and their towing capacity ranges from 2000 lb to 5000 lb depending on the model. I owned a Highlander prior to the 4Runner and towed a popup weighing 2000 lb all over the USA. It did a great job. But I wouldn't even think of going with something over 3500 lb with it. It was also a V6, and mine was rated for 3500 lb, so my little popup was well within specs. What I learned when I wanted to upgrade to a TT is that only about 15% of Highlanders have the towing package, and it is only with the towing package that they are rated for 5000 lb towing. That's why I got rid of it and got the 4Runner.
As has been said, the overall payload is the most constraining limiting factor on these SUV's. You will be running into that limitation before you know it.
-Speak
