It sounds like the guy had some extra 4-wire laying around and used it instead of buying the right wire? You need to open up the plug and open up the welder to see how it's wired. If it's single phase you should be on Black for power, White for common, and Green for ground if I'm not mistaken. Make sure the plug and breaker and gauge of wire are correct for the amperage. Cut the extra wire back on both ends and tape it up.
Congrats on the new purchase and have fun! Wish I had one in the garage as well!
