I figured that 4 additional seats has to mean either 3x1, 2x2, or 1x3 since 4 seats can't fit on one side of the aisle. That means another row, even if it's adding seats on one side toward the front of coach and on the other side toward the rear of coach.
Being that the A321 doesn't have overwing exits, they can't do the 3 seats in every row and just space the overwing exit rows a little further apart instead of leaving a seat out of rows between overwing exits. So something has to give - pitch, type of seats, take out something to make room. After all, they can't move the exit doors or rear bulkhead to make more room. The only other alternative, which wasn't mentioned as being a part of the change, would be to remove a row of FC but that wouldn't add 4 seats to overall capacity.
Jim