Amazon.com: Oyviny 4-Way Flat to 7-Way Round Blade and 4 Way Trailer Wiring Harness Adapter Vehicle-Side 4 Pin to Trailer-Side 7 Pin and 4 Pin Dual-Output Connector with Mounting Bracket for Truck
4 Pin to 7 Way Blade RV Trailer Wiring Adapter Connector Cable Plug w/Bracket | eBay
Reese Adapter 7 Way Round to 4 Way Flat in the Trailer Parts & Accessories department at Lowes.com