I just purchased a 6mm 60quart pot. It is a monster. I havn't used it yet because I don't trust it with 10 gallons on my turkey fryer. It's not a "bayou" type and is a bit flimsy. I'm in the process (thinking about) fabricating a new stand. The (2mm?) pot that came with the fryer works well enough for 5 gallons, extract.
I have not seen a 4mm but I'm not sure 6mm is essential. It's certainly sturdy, and I don't think it will ever dent. Here's the one I got.
The shipping was around $20, so it came out to just over $100.