You have a "migratory bird plug" in the mag tube to limit the gun to 3 total shells for waterfowl and other migratory birds...
Simply remove the barrel retainer screw and shake the thing pointing down. if a little rod don't poke out, try pushing three shells in, that should push it out enough to grab with fingers and pull out...
http://www.youtube.com/watch?v=zwNCQhVdGgw
this is a link to the tutorial for this...
Worst case requires peeking in the hole and use a thin wire to guide it out...
Then you get 5+1 capacity...
Brent