Taillight wiring/Converting pickup to a trailer ?

I have a ratty old longbed truck that I'm converting into a trailer. My question is in regards to the taillight wiring. I'll be using a 3&1 4 pin connector. Am I to guess that one wire is for the left light, one is for the right, one is for brakes and the solo one is a ground?
Thanks, Greg

http://www.etrailer.com/faq-wiring.aspx

You are almost correct.

Green - right turn
Yellow - Left turn
Brown - marker & tail
White - ground

The turn signal light circuit also functions the brake lights.

Thank you so much.