Fourth umpire sometimes referred to as the reserve umpire

Umpire (cricket)

In cricket, an umpire is a person who has the authority to make judgements on the cricket field, according to the Laws of Cricket...

, is a cricket

Cricket

Cricket is a bat-and-ball game played between two teams of 11 players on an oval-shaped field, at the centre of which is a rectangular 22-yard long pitch. One team bats, trying to score as many runs as possible while the other team bowls and fields, trying to dismiss the batsmen and thus limit the...

 official. For all international matches a fourth umpire is required to perform duties like bringing on the new ball, carrying drinks on to the field for the umpires and observing the pitch during the lunch and tea intervals to make sure there is no interference. The fourth umpire will take over the Third umpire

Third umpire

In international cricket matches the third umpire is an off-field umpire who makes the final decision in questions referred to him by the two on-field umpires. Television replays are available to the third umpire to assist him in coming to a decision...

duties if something happens to one of the onfield umpires, as the rules state the third umpire will take over on field duties.