The Football Association have confirmed that teams can use a fourth substitute in FA Cup matches.
However, this only applies in extra time from the First Round proper onwards.
The FA have said: "In accordance with Emirates FA Cup competition rules, a maximum of seven substitutes may be nominated for a game, and a club may, at its discretion, use three of those seven substitute players at any time in a match during normal time.
This season, in the event that extra-time is played in any match in the competition proper, a club may use one additional player from the nominated seven substitutes during the period of extra-time."