Here is the rule on that.

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. Nobody has an option of accepting or rejecting the card. The betting is then completed. The error is rectified in a manner to least influence the identity of the boardcards that would have been used without the error. The dealer burns and deals 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 deals 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. [See Chapter 16 - Explanations, discussion #2 for more information on this rule]