Five Wire Trailer Diagram. Better yet add a couple of amber LED tail lights and wire the trailer for five pin. No adapter needed then, and you will have better lighting on the back.
The red and blue wire can be used for brake control or auxiliary. This is accomplished through either a T-One connector (if available for your vehicle) or through hardwiring. A brake controller has only one output wire.
Trailer plug connector diagrams for electrical towing connectors.
This page has wire diagrams for many electric options including wires for trailer lights, brakes, alt power and connectors.
To provide the power and a hook-up for these, the tow vehicle's wires are tapped into. No adapter needed then, and you will have better lighting on the back. In the North American market it is very common for brake lights and turn signals to be combined.