The black and red wires are the two 120 volt lines, typical of 240 volt operation. The white is the neutral which is necessary if the welder has any 120 volt devices (like a kitchen range might have a clock or an oven light which was 120 volt). The green is the equipment ground. You can't safely, or legally, install a plug on that wire which will plug into the outlet you have. You also can't legally change the outlet to one which will accomodate the plug that should go with that wire because then the wiring from the outlet to the breaker panel will be wrong.
The advice from Lowe's "electrical guy" is clever, but it's inadequate, and probably also illegal (code violation). Everlast tech support is unlikely to be able to help you if they ever return your call. With no disrespect intended toward you, I'd say you're in over your head on this one. You need to get an electrician to take care of this for you. Illegal wiring gives you parents' insurance company an easy out if there's ever a fire.