4" 321 stuff is going to be kind of hard to come by. The only thing it is generally used for is turbo header applications. There usually isn't anything before the turbo that is that big. Once you get past the turbo the exhaust isn't nearly hot enough to need 321. It would be a waste of money since 304 works just fine.
Just out of curiosity what is he using it for?
This place has it, but not a lot of 4" 321 stuff is not listed on their site. You'll have to email or call them.
You might also try emailing or calling some of the bigger companies too. I'm sure a lot of them can get it. I have found that Columbia River Mandrel Bending seems to have about the best prices for 304. I'd get ahold of them and see if they can get you some 321.