Bill, first off; yes, your WSM will easily hold 4 6lb butts. Either 2 on each grate, or I would think if you placed them on their sides, you may be able to fit all 4 on the top grate. My question is; is time your concern on these cooks? We each have our own favorite methods and techniches, but if you were to go with a dry foil covered water pan, and increase your temp from *225 to *275, you would be able to reduce the cook time by +/- 6 hours.. I have smoked 2 8lb or 9lb butts at *275 and I'm always in the 10 hour range. Of course every cook is slightly different, and nothing in this wonderful addiction we call
Q-ing is absolute, but with my target temps between *265-*275, my Boston butts are pretty reliably in the 10 hour range.
Hope this helps,
Tim