A big one (LOL). Sorry martyrd, but I ahd to pull that one on ya.
Seriously, when I go to the range to pattern a shotgun or sight in a rifle or pistol I use a large piece of construction paper. I get it at the builder's supply for about $6 a roll (large roll) and the stuff lasts forever. It's about a 3 foot long roll and I usually take about a 3'x5' section, staple it and paste on one of those shoot and see targets as an aiming point.
I also try and draw something of a properly concentric circle on it and quarter the circle so I can have some impression of where the majority of the pellets are hitting.
With #4 buck in 12 guage I think you have 27 pellets, so its sholdn't be too difficult to see what your pattern is like. HTH. Mikey.