Here's the deal. I'm making a jumper harness to go from my 6 KW generator to my arc welder. I have the plug ends, but one is a 4 wire connector twist-lock connector and the other is a three wire plug-in. Presently I use the generator to supply emergency power to the house, but figured if I just made a jumper, then I could just plug into the generator for portable power to the welder. Problem is....what do I do with the extra wire?

Edit: I guess I should explain the photo: The black plug will connect to the welder, the white one to the outgoing power cord on the generator. Thanks