The Celtics started hot and continued their high-pace scoring throughout the game. Smart was missing after injuring his ankle in Game 3 - with Derrick White starting at point guard.
Heat started 0/14 from the field as they were down 18-1 after a horrendous opening quarter. Boston won the first by 18 points and the second by six as they went into the half up 24. Miami were down by 20+ the whole second half until 03:36 was left on the clock in the fourth when they made the deficit 18 points.
On an extremely disappointing night for the Heat, their starters combined for a shocking 18 points - the fewest by a starting five in the playoffs since this stat was first tracked in 1971. With the chance of victory extremely slim, Erik Spoelstra emptied his bench as he looked ahead to Game 5 early on.
Victor Oladipo scored 23 off the bench - eclipsing every starter combined. In less minutes on the floor than Jimmy Butler (6 points) and Bam Adebayo (9), Caleb Martin (12) and Duncan Robinson (14) both scored more points. They were without Sixth Man of the Year, Tyler Herro, who had a groin problem.
It was a solid night for the Celtics' starting five, with Tatum leading all scorers with 31. Thankfully for Ime Udoka, the absence of Smart did not hinder his side. His replacement, White, scored 13, and had eight rebounds, six assists, one block and three steals. Jaylen Brown was allowed some rest time as he scored 12 in 29 minutes.
Spoelstra downplayed his team's heavy defeat, saying: "Whatever they've done to us, we can do to them. None of us are happy about what happened. There are these extreme highs and lows particularly when you have 2 teams pretty evenly matched.
"They came out and jumped us. We're not making any excuses. They outplayed us tonight for sure. We never could get any kind of grip on the game."
Adebayo said: "It's a part of playoffs. You learn to adapt. Guys being out, guys playing half, guys playing 20 minutes in the game, just depends. You've just got to find a way to win."
Tatum: "Obviously, we knew how important this game was. Everybody just had to come up with a different sense of urgency."
After improving to 5-0 following a loss this post-season, Udoka said: "Just got to muster that same energy when we came off a win as well as a loss. This is a three-game series now. Can't always just flip the mindset when we come off a loss and get a little desperate."
Game 5 is on Wednesday in Miami.