DF,

I recently ran into a similar 3-prong, 4-prong situation. I'm pretty sure you have it correct. White = common/neutral is connected to the same buss bar as the ground. What I has was: red = hot; black = hot; white = ground, common/neutral = all the same; green = ground.

I was going to wire a 4 to 3 prong pigtail, but talking with a guy at Lowes who used to do lots of these electrical installations (according to him), he said to change out the cord to a 4 wire cord, and have the white and green go to separate grounding locations on the appliance (not a welder). But I checked and both bars that I grounded to on the appliance were connected, i.e., common ground.

I have no idea why, but it's due to changes in the electrical code.

Sg