FIFA.com - The Laws of the Game - page 53
A fourth official may be appointed under the competition rules and officiates if any of the three match officials is unable to continue, unless a reserve assistant referee is appointed. He assists the referee at all times
• Prior to the start of the competition, the organiser states clearly whether, if the referee is unable to continue, the fourth official takes over as the referee or whether the senior assistant referee takes over as referee with the fourth official becoming an assistant referee
• The fourth official assists with any administrative duties before, during and after the match, as required by the referee
• He is responsible for assisting with substitution procedures during the match
• He has the authority to check the equipment of substitutes before they enter the field of play. If their equipment does not comply with the Laws of the Game, he informs the referee
• He supervises the replacement balls, where required. If the match ball has to be replaced during a match, he provides another ball, on the instruction of the referee, thus keeping the delay to a minimum
• He assists the referee to control the match in accordance with the Laws of the Game. The referee, however, retains the authority to decide on all points connected with play.
• After the match, the fourth official must submit a report to the appropriate authorities on any misconduct or other incident that occurred out of the view of the referee and the assistant referees. The fourth official must advise the referee and his assistants of any report being made
• He has the authority to inform the referee of irresponsible behaviour by any occupant of the technical area
• A reserve assistant referee may also be appointed under competition rules. His only duty shall be to replace an assistant referee who is unable to continue or to replace the fourth official, as required