Obviously there aren't 4 million rugby supporters in New Zealand; I'm sure we all know people who aren't interested in rugby (and some who even hate it), but the World Cup will be well supported and generate significant interest. The majority of New Zealanders never go sailing, but Auckland was pumping during the America's Cup and people with little or no knowledge of sailing became keen enthusiasts during the regatta, because it was New Zealand on the world stage.
Having said that, I doubt it will generate as much revenue as the last World Cup, or, in fact, if it were held in Japan. You won't get 30,000 at a Uruguay vs. Georgia game -- not paid in full. What you may get, however, is New Zealanders adopting the underdog, something you probably wouldn't see in Japan, who'd paper the game.