I Have A New 4500 And It Did Not Come With The Plug For The Trailer Brake Controller Like My 3500. I Looked Under The Dash And There Is Not A Port For The Plug Like The 3500. There Is A Totally Different Round Plug That Looks Like It May Work. Where Do I Get This Plug Assuming That It Is This Round Plug.

Contact the manufacturer of your brake controller. Even if you don't have plug/adapter, you may have a harness bundled together somewhere in the dash, like my old Trailblazer did.