Do i have to pay off my car before i trade it in and finance through someone else?

i currently have a loan left of $4000 or so on my car...owe a little more than its worth, but i want a new car and NEED my co-signer off of my loan. How do i go about going to a used car lot...finding a car i like...trading in my current car and re-loaning? or does it get added to my current loan? ehhh im confused! ? help!