The point is that at 400 yards, or 1/4 miles, or across a 40 acre field, which are all about the same distance, the amount of cannister which would arrive on target could not have been as effective as a good volley from a regiment of 400 men. A battery of 4 guns would have had to have fired a cannister shot of 100 balls per piece to achieve the same volume of fire. Again are you using a 3 inch smoothbore or rifled gun the most commom piece on most battlefields, or 12 pound howitzers or Napoleans. As with any "shotgun" pattern, as has already been pointed out, you will have "flyers" from that shot pattern. Some pellets will go high, some wide, and as been pointed out some will hit the ground and skip losing energy and range. The effective percentage of shot the fly true will probably be about 60%.
At 400 yards the infantrymen armed with rifled muskets would in my opinion have the advantage in that they would be aiming at just 4 compact targets as they fired on the gun crews, while the gun crew would be aiming at a mass that extended across a 200 yards, or 1/8 mile wide front. I doubt that the spread of cannister in a 4 gun battery especially of 3 inch weapons would cover the entire 200 yard regimental front of a 400 man regiment at 400 yards distance. Then add into the mix other factors such as land is very rarely completely flat across any battlefield and if the Infantry took any evasive actions, cannister effectiveness would, in my mind, be seriously in question.