Fourth street is a poker term used to refer to a specific stage in a round of poker.
In a game of poker, apart from receiving their own cards dealt face down, players can often make use of the community cards, depending on the variance of poker they are playing. Community cards are dealt face up and are displayed in the centre of the table. Players can use them in combination with their cards to form winning hands.
If you are playing Texas Hold’em, during the third round of betting there will be four community cards on the table. For that reason, many poker players refer to the third round of betting as the fourth street. However, keep in mind that some players may refer to that round as the turn as well.
If you are playing a game of Seven-Card Stud Poker, for example, during the second round of betting the players will have four cards in their hand — two hole cards, a face-up door card, and a face-up fourth card that poker players call the fourth street.