Depends on exactly what happened - did they just deal a four-card flop by accident, or did they burn and turn the flop prematurely?
The general rule is that you correct the error in the manner that has the least impact on what the actual board cards would have been. Here's the relevant section of Robert's Rules: http://www.homepokertourney.com/robe...ION_5_-_HOLDEM
If it was a four-card flop dealt accidentally (ie: only one card had been burned) then the fix is fairly trivial: you take the extra card off the flop*, make sure everyone has seen it so they all have the same information, then use that card as the burn card on the turn (which is what it was supposed to be anyway) and carry on with the game normally.
If the turn was burned and turned prematurely then there's a procedure in the above link for correcting it. Basically you take the turn card off the board (but leave the burned card where it is - it's in the correct spot), finish the betting round, burn and turn again
(so the cards that would have been burned and turned for the river
effectively become the turn cards). Then reshuffle the prematurely dealt card into the deck stub and deal the river without
burning a card.
* make sure you take the correct one though - if the dealer has followed the standard method of spreading a flop, it'll be the card on the far left of the board that shouldn't be there