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I don't think towing that much on any Rav4 is a very good idea. Going 500 pounds over is one thing almost doubled is another. The TRD and Adventure do get a 3500 pound tow rating so 4000 wouldn't seem that far off but you also run into payload ratings as well. 10% of the weight onbthe tounge is not enough in my experience especially when you use a weight distribution hitch (which you should with that much weight) as the hitch will move some of the weight back to the trailer axle as well as the front axle of the truck. So to get it rock steady at 65 mph+ downhill, windy, with cars passing (because that's when sway will suddenly sneak up and bite you) I'd run at least 12% but shoot for 15%. Now also consider any WHD hitch head and bars with sway control will also add probably 70 more pounds puts you at 580 pounds hanging off the unibody rear. (The TRD only gets 1050 pound rating) So just a driver and a passenger without luggage will be fine, but you see lots of people running both the car and trailer packed and not many things are designed to do both haul and tow.
That said, don't look at airbags, see if Timbren makes a load helper for it. They are great, they replace the suspension stop bumper with a bigger heavy duty peice. Unloaded it doesn't touch the harpoints so the ride is 100% factory, as soon as you add some weight it starts helping increase the resistance progressively getting harder and harder. But it never leaks, never needs adjusting, is always ready, and the last set I put in took no tools but a jack and 10 mins. Not all go in that easy though, I also put them on a 2500 Chevy Duramax and that needed a bracket bolted on. Both sets were under $300 if I remember.
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