The experiment will begin when the first round takes place on Saturday 24 September.
It is an initiative being introduced by the Scottish FA after recent talks with the International Football Association Board.
Previously, teams could only use three of seven substitutes in Scotland's premier cup competition.
The fourth replacement will only be available if a tie is level after 90 minutes and goes to extra time.
There are no plans for the format to be introduced by the Scottish Professional Football League, but it could be used in next season's League Cup in Scotland.
The SFA announced the extra substitute plan as Hibernian captain David Gray made the first-round draw at Edinburgh College.
One preliminary-round tie remains to be played without the availability of an extra substitute after Scottish Junior Cup winners Beith Juniors and West Superleague champions Auchinleck Talbot drew 2-2 at Bellsdale Park on Saturday.
Talbot host their West Superleague rivals this coming Saturday with Highland League outfit Strathspey Thistle now preparing for a journey south to face the victors.
East Superleague champions Bonnyrigg Rose and North Superleague champions Banks O'Dee both face a trip to the Highlands to face Turriff United and Keith respectively.
East of Scotland champions Leith Athletic host Cumbernauld Colts, who finished fourth in last season's Lowland League.
Beith Juniors or Auchinleck Talbot v Strathspey Thistle
BSC Glasgow v Rothes
Civil Service Strollers v Hawick Royal Albert
Clachnacuddin v University of Stirling
Dalbeattie Star v Wick Academy
Deveronvale v Gretna 2008
East Kilbride v Vale of Leithen
Edinburgh University v Whitehill Welfare
Forres Mechanics v Lossiemouth
Fort William v Brora Rangers
Gala Fairydean Rovers v Fraserburgh
Girvan v Huntly
Inverurie Loco Works v Buckie Thistle
Keith v Bank's O'Dee
Leith Athletic v Cumbernauld Colts
Nairn County v Preston Athletic
Selkirk v Linlithgow Rose
Turriff United v Bonnyrigg Rose
Ties will be played on Saturday 24 September.

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Her team face the Netherlands in Friday's final looking to follow Sean Kerly's side, the champions in Seoul.
"You saw what happened to hockey in the country after that - it really picked up," said Richardson-Walsh, part of a team who won bronze at London 2012.
"It picked up after London too, and hopefully the same will happen again."
Britain's women are chasing their first gold, 28 years after the men beat West Germany 3-1 to win their second and most recent Olympic title, with two goals from Imran Sherwani and one from Kerly.
"I remember my parents getting up to watch the 1988 final, but I was only eight - I was a little too young," Richardson-Walsh said.
Friday's final, which starts at 21:00 BST, will be the 36-year-old's final game after playing 374 times for Great Britain and England.
She will also be playing alongside partner Helen - the pair married in 2013.
"It will be really special to be in a final with her," said the captain.
"We have been through ups and downs - some of the best and worst times over the last few years.
"For Helen to come back from back surgery twice, to get over that and play again and reach an Olympic final, it's great."
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A special service was held at St George's Hall on Wednesday evening to remember the 96 football fans who died in 1989.
These people tell us why they came to pay tribute.
Two of the messages...
"I pray that all those who were lost in the Hillsborough accident are always remembered and respected. I also pray that the innocent fans who got the blame will get justice."
"We will never forget you. You will never walk alone."
Libby says: "Everyone is overwhelmed and so happy about the outcome. They want everyone to know that it wasn't the fans' fault."
Joe says: "He's always cried because it's such a touching story."
Zac says: "They've been waiting for justice for a long time and they've finally got it... It's important for them to never forget about what happened."
A fourth substitute will be available in extra time for the first time in this season's Scottish Cup.
[NEXT_CONCEPT]
Great Britain's women can reinvigorate hockey at home if they emulate the 1988 men's side by winning Olympic gold, captain Kate Richardson-Walsh believes.
[NEXT_CONCEPT]
Children and teenagers have been among the thousands of people remembering the Hillsborough disaster victims in Liverpool city centre.