The 4% increase is merely a numbers game.
First, they round up. If my math is correct (and someone can check it for me) it was actually a 3.39% increase, not a 4% increase in average publishers in the USA.
Second, the 2009 service year was the first one (and again, correct me if I'm wrong) in which elderly and infirm publishers could count as little as 15 minutes in a month and still be considered (read: counted for the stats) as an active publisher.
Third, it may have also been the first service year in which (once again, someone correct me if I'm wrong) BOTH parents could count their time while studying with their children, instead of just one.
These things allow for faders to keep reporting, too, remember, if they have kids or if they happen to be old or in some way ill or injured.
Taking all this into account, and looking at the 2.5% DEcrease in USA baptisms (from 35597 down to 34739, is my math right?), the 4% increase has to come from the new methods of gathering the statistics. It isn't coming from new Jehovah's Witnesses.