[undy]

I'd get a sawzall and cut through 70% of the diameter of #4 pipe on the distorted side upward, as close to the flange as possible. I'd the get a "fat tipped" oxy/acet torch and "cherry red" that distorted flange. Put the header flange flat on the concrete floor or driveway (large anvil preferred) and flatten #4 flange, comparitive and equal to the others, working it with a hammer. Once you get it where you want it dress it up with a file. An auto machine shop head resurfacer (sander/abrasive type) would be better and get you a 100% flat flange. Once you have the flange where you want it tack weld it to the tube. Recheck flange with a straight edge and reweld/fill the gap between the flange and the tube. You're not going to see the repair once it's installed in the car so it doesn't have to be real pretty, just effective.

Dave