since you already have the 4 pin connector wired, this may be the way to go. http://www.reese-hitches.com/products/,LM-47185
It has a pigtail that plugs into your flat connector, plus features the 7 pin round connector and the flat connector both to plug a trailer into. You can buy them around $25 at most anyplace that sells trailer lights, wiring, etc. Even down to places such as Autozone and Walmart.