We towed our loaded CD 22 all over the West with a 1989? Pathfinder, rated for only 3500-4000? lb, with no problem at all. We had the higher-grade suspension, which definitely makes a difference. It actually worked a little better than our 1994 Trooper, the rear springs of which were too soft. Guess we could have added air bags to the Trooper, but didn't try that.
Getting the tongue weight right (not too light: 7-10% of total trailer weight) is key to avoiding sway and keeping the trailer under control. For the tow vehicle, more weight, longer wheelbase, adequate suspension with good shocks, and rear tires pumped to the max rated pressure are keys. Pump the trailer tires to max as well.
The 4-runner should work well, if set up correctly (tongue weight, tire pressures, etc).