As per Robert's Rules of Poker
http://www.diamondcs.net/~thecoach/RobsPkrRulesHome.htm,
If the dealer turns the fourth card on the board before the betting round is complete, the card is taken out of play for that round, even if subsequent players elect to fold. The betting is then completed. The dealer burns and turns what would have been the fifth card in the fourth card’s place. After this round of betting, the dealer reshuffles the deck, including the card that was taken out of play, but not including the burncards or discards. The dealer then cuts the deck and turns the final card without burning a card. If the fifth card is turned up prematurely, the deck is reshuffled and dealt in the same manner.