The boat trailer most likely has surge brakes, which will require no electrical connection to work.
The trailer tongue has a mechanism in it that slides back and actuates a plunger in a master cylinder, pushing hydraulic fluid into the wheel cylinders, actuating the brakes.
If your 4Runner has an automatic, a cooler is a mandatory option. Even so, I know of one Toyota Automatic in a 'runner that failed less than 25 miles from home pulling a 4k boat. Engine braking with an automatic in a 4Runner will be negligble.
Hopefully you have a 5-speed, which will be marginally better. I don't know if you have pressure and return ports on a 5-speed for a cooler, I doubt it.
Braking, as has been mentioned, is going to horrible; be very, very careful. When you can, upgrade your tow vehicle.