There is no reason a 4 wheel trailer must be unstable. As has been mentioned ( in British English) multi- trailer semi trucks are done at freeway speeds.

There are some practical considerations that hurt stability:

-The bed often has to be quite high to clear the steering motion of the front wheels. Horse drawn wagons often have smaller front wheels than rear so as to lower the weight.

-Tho tongue on simple solid axle steering needs to be fairly long, or the steering ratio becomes too high. A kingpin arraignment can use the lever ratios to slow the steering

One big problem not mentioned is braking. A two wheeled trailer adds weight to the rear wheel of the tug, increasing traction. A four wheel Trailer has zero tongue weight. Without brakes on the trailer, it will be easy to skid the rear wheel, which is likely to jack-knife the combo. Even if the rear brake is not used, enough weight shift can happiness to cause th rear wheel to become light enough to lose traction.