The rule will be trialled in Brazil during the men's and women's tournaments
Fourth substitutions could become a regular feature during extra-time of football matches if a new trial is to become law.
FIFA have confirmed that they are to allow the trailing of the new rule at numerous tournaments in 2016 including the Olympic Games and the FIFA Club World Cup.
The International Football Association Board changed the rules of the game earlier this month in Cardiff which will see if the rule change will help player welfare or whether the fourth sub will be used tactically.
"Following the recent decision by The IFAB to allow experimentation with a fourth substitution in extra time, the FIFA Executive Committee agreed for such experiments to be conducted this year at the Olympic Football Tournaments, the FIFA U-20 Women’s World Cup Papua New Guinea and the FIFA Club World Cup Japan."
The rule change comes into effect on June 1st. FIFA will use the four tournaments to see of the rule change is given permanent status in FIFA tournaments.