For 4 people, you could easily get away with a 6 or 7# Boston Butt. If you have a choice, try to get the bine-in butt rather than boneless, although the last one I did was a 9# boneless that worked out fine, but I tied it up with butcher twine to avoid having it fall apart. Take a look in the PORK forum fro a nice brine recipe that Divotmaker provided...I used this with great success! A brine is not a necessary component for a butt since it has lots of fat that will render during the smoke, but if you have time, I would recommend trying the brine. I brined for 12 hours, then rinsed, dried and applied a rub for another 12 hours before the smoke.
Boston butts take a long time to smoke, so give yourself at least 2 hours per pound for the smoke (6# butt could go for 12 hours). Use 5-6oz of your choice wood, and insert a remote meat therm in the largest part of the butt without touching the bone (if you go bone in). I set the smoker for 230F, but be forewarned that the butt will start coming up to temp quickly and then around 165IT or so, you will experience the "stall" where the butt will stay at the same temp for hours...don't panic, just let it keep smoking and after a few hours you will see the temp start to increase!
For pulled pork, you will want to get to 200-203F before pulling the butt from the smoker, then double wrap in foil for at least 30 minutes before pulling. It's a long smoke, but the wait will be well worth it!
If, on the other hand, you want to just do sliced pork rather than pulled, you can remove the butt from the smoker when you hit about 175IT.
Good luck!