4-Pin vs 7-Pin Trailer Wiring: What the Extra Pins Carry
Four pins carry lights. The extra three carry trailer brakes, reverse and a charge line, which is why a braked trailer cannot use a four-pin connector.
Brake Controller Wiring: Plug-and-Play vs Aftermarket
If the harness plugs into an existing connector, it is an owner job. If it involves the brake switch or a battery lead, it is brake-circuit work and it is a shop.