A 4-way or 7-way trailer connector is labeled as such based on the number of pins it has. The pictures I've attached show the two. You can see the 7-way has 7 pins, 6 of them in a circle pattern and one more in the center. A 4-way has 4 pins straight across.
A 4-way connector controls the trailer's lights; one pin for the left side brake lights and turn signals, one pin fo the right side brake lights and turn signals, one pin for the running lights, and finally a ground pin to complete... view full answer...