There are 7 wires in your trailer wiring.
2. left turn
3. right turn
(brake and emergency flashers use both the left and right turn wires together)
4. tail lights, marker lights
Those four wires are all that is in the "U-Haul" 4-flat-wire connectors.
5. trailer brakes: this wire is "hot" only when the trailer brake controller is sending juice to the electric (magnetic) trailer brakes
6. 12-volt hot = Battery charge = 12 volts (nominal) hot all the time the key is on.
7. Auxiliary = not normally connected to anything, but can be used for backup lights, porch lights, etc. This one is normally also "hot" all the time the key is on, unless the wiring has been modified to use backup lights.
Trailer Wiring Diagrams | etrailer.com