With GM anyway, Ive seen alot of them with 6 lug converters, most were diesels, and you can get new converters that are 6 lug, I've seen them mainly for the tractor pulling crowd. The 4 bolt they're talking about is normally Ford/Mopar.
It's TCIAUTO.com
PRO-X™ 9" & 10" Race Converters
The TCI® PRO-X™ 9 & 10-Inch Race Converters are designed specifically for Top Sportsman, Top Dragster and Outlaw Street Car classes. Able to handle 2500 hp plus applications (including multi-stage, nitrous assisted, supercharged and twin turbocharged engines), these high strength torque converters feature a TCI® exclusive - HDT Coating™ (Heat Dissipating Technology™) - further enhancing their ability to deliver unsurpassed consistency and durability.
Features:
Hand-built steel stator
Forged mounting cover
3 sets of heavy-duty internal bearings
Furnaced-brazed turbine and pump assembly
Built for GM, Ford and Chrysler transmissions; six or four-bolt flanges and optional billet front available