Your 4Runner is already setup from the factory with both 4-Pin and 7- pin plugs under the rear bumper. These are both standards for the industry- so just plug your trailer harness into one of these and you are good to go!
As mentioned the 5th gen has the plugs. However, they are not real obvious. You almost need to be looking up at the undercarriage to see the flaps that cover the two plugs (4 pin and 7 pin). After you know where they are located it is easier.