The 4th official holds up the substitution board during MLS Cup 2016
USA Today Sports

Fourth substitution used for the first time in MLS during ATL-CLB match

Back on August 17, MLS unveiled a new rule allowing clubs a fourth substitution for matches that go into extra time during the Audi 2017 MLS Cup Playoffs.

We saw it in action for the first time on Thursday night as both Atlanta United and Columbus Crew SC made use of the extra sub during the extra time period that followed their 0-0 regulation draw. The single-elimination Knockout Round match proceeded to a penalty kick shootout, which Crew SC won 3-1.

Columbus forward Adam Jahn, who clinched the shootout victory for his club in the fourth round of the shootout, came into the match in the final minute of extra time after his club had already made its first three subs. 

Just six minutes prior, Atlanta United made its fourth substitution of the match when forward Jacob Peterson came on for Josef Martinez.

The International Football Association Board (IFAB), which oversees changes to the Laws of the Game, approved experiments for a fourth substitute in extra time. Under the IFAB regulations, a fourth substitute may only be used if a game goes to extra time and may be used regardless of the number of substitutes a team made during regulation time.

Fourth substitute experiments have already been run in the FA Cup, Scottish Cup, FIFA Confederations Cup and FIFA Club World Cup – a final decision on the incorporation of a fourth sub will be made by IFAB in 2018 or 2019.

Matches tied after regulation throughout the Audi 2017 MLS Cup Playoffs move to two, 15-minute extra time periods. If the tie isn’t broken in extra time, the series or match is decided by kicks from the penalty mark.