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.

I have checked the trailer, and I am now sure that it has "some kind"
of brake. It surely looks like it is a surge-brake as what you have
suggested because the trailer coupler is quite oversized.

The other thing is that the wiring actually has 5 wires instead of 4
wires as what I said previously. I guess my memory was fooling me.
The wiring plug totally has 4 metal prongs. Two of the wires goes into
the same prong.

Anyway, now that I know my trailer has a surge brake, I don't need to
worry about submerging the trailer into sal****er (as long as I wash it
afterward). Great!

Thanks.

Jay Chan