The Hoops were trailing 1-0 at the break following Daniel Majstorovic's early own-goal but hit back strongly in the second term as 20-year old winger Forrest scored twice before the hour-mark and then Anthony Stokes and Gary Hooper got in on the act.
Manager Neil Lennon revealed after the match that he asked Forrest to change his boots at half-time and in the end it did the trick as Celtic progressed through to the semi-finals after the talented Scotland international kickstarted things at Easter Road.
"I told Forrest to change his boots for a start," Lennon said.
"He was playing with these white moulded boots on and he kept hitting the first man, stumbling over the ball and wasn't right in his thinking at all.
"James is the type of kid you can have a go at and get a positive response and he set the tone for the second half straightaway, he was magnificent.
"He is a great talent, no doubt about it, he is very quick over five or 10 yards, the physical side is improving and he has a lot of belief in himself.
"He goes both ways with pace, he plays with menace he is a hard man to stop and he will only improve."