He may be appointed under the rules of a competition and shall officiate in the event of any of the three match officials being unable to continue.
The organiser, prior to the start of the competition, shall state clearly whether, in the event of any referee being unable to continue, the fourth official would take over as the match referee or whether the senior linesman would take over as referee with the fourth official becoming a linesman.
The fourth official shall assist with any administrative duties before, during and after the match as required by the referee.
He shall be responsible for assisting with substitution procedures during the match.
He shall supervise the replacement footballs where required. If during a match the match ball has to be replaced he shall, on the instructions of the referee, provide another ball thus keeping the delay to a minimun.
He shall have the authority to check the equipment of substitutes prior to their entering the field of play. In the event of their equipment not being within the Laws of the Game he shall inform the linesman who shall then inform the referee.
The fourth official will assist the referee at all times.
FIFA Handbook | Laws of the Game