Sounds like it is like most EZ-loader trailer. Surge brakes. The
coupler where you attached to the tow ball slides and activates a master
cylinder to provide braking. The 4 wire plug has a ground wire in the
rubber shielded connector and the other 3 are running lights and brake /
turn signals for left and right.

Thanks for the pointer.

I will have to get under the trailer to see if the trailer has a brake
or not. If I see a brake, I will assume that the trailer has a surge
brake because the wiring only has 4 wires. Does this sound right?

Jay Chan