The pattern master is suppose to shorten the shot string. This is good because you either have alot of pellets on the target at once or you miss completely (at least in theory). It also keeps it tight. If you stay in close though and use smaller shot, even 6's on occasion, you can have a longer shot string, sufficiant pellets, and a little more room for error.
There was a very in depth study performed with the results being the "CONSEP 2000 Steel shot Lethality Table". It states " steel 4s has exhibited good all-around performance for taking small and medium ducks,but has not proven lethal on large ducks beyond 45 yards." It also states steel 3's are the best all-around performance for taking ducks.
The main thing to remember is you want the most pellets (smallest size) you can use that will completely penetrate the bird at the distance you will shoot. If you stay within 40 yards 4's are hard to beat. If you may, on occasion, have to shoot a little farther 2's or 3's will still get the job done just with alot more holes in the pattern.