I just get annoyed when game changing mistakes happen when the 4th official could make a call but doesn't. It happens in too many matches.
What has this got to do with the Fourth Official? The Fourth Official's duties are clearly laid down. He can only assist the Referee, the Referee's decision is the one that matters and it is final (this is also laid down in black and white).
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
Extract from the Laws of Association Football
http://www.thefa.com/my-football/referee/laws-of-the-game