Game 4 was like the Celtics’ season: Just when you started to count them out, everything changed

Maybe we should have known the Celtics would find a way to put up a furious fight in the fourth quarter.

That they would somehow muster their best when circumstances seemed to be accelerating toward the hopeless.

That they would not let any early, prolonged frustrations devour their determination before everything was at stake later on.

After all, what the Celtics achieved Monday night in Game 4 of their Eastern Conference semifinal series with the defending champion Milwaukee Bucks was a fairly obvious microcosm of their season.

Just when it was getting tempting to abandon them, the Celtics turned into something special.

Trailing by 7 points entering the fourth quarter and staring down the barrel of a 3-1 deficit in the series, the Celtics got an all-timer of a performance from Al Horford, a superstar-level “I’ve got this” takeover down the stretch by Jayson Tatum, and one relentless performance after another from assorted others in a stirring 116-108 victory at Milwaukee.

The Celtics outscored the Bucks, 43-28, in the fourth quarter, hitting 16 of 19 shots after making just 26 of 65 through the first three frames.

They attacked again and again, scoring 48 points in the paint on the night and refusing to be intimidated in this heavyweight bout against the current title-holder.

And they kept their composure and confidence even when Giannis Antetokounmpo, the best player in this galaxy and presumably a couple of undiscovered others, flipped the switch to unstoppable mode.

“For us, it was just keep going,” said Marcus Smart, who scored 18 points, dished out eight assists, and backed down into the paint for two important buckets in the fourth quarter. “[At the end of the third, we thought] adversity is here. We have to get over that hump. We have got to do whatever we can.

“Through all the mistakes we made, buckets they hit, free throws we gave up, turnovers, and the game was only a 7-point game. That was very encouraging on our end because we knew we could play better. We knew that once we did play better, things would be turned around for us. And that’s what we did. We stayed to the game plan, we stayed together, and we continued to fight.”

As fitting as it might have been as a single-game parallel to their entire season, this win must not be taken for granted. What the Celtics achieved Monday night might have been in their character, but it was also singularly remarkable.

Particularly the extraordinary, explosive performance of the oldest and supposedly most mild-mannered Celtic.

Al Horford has played 15 seasons in the NBA. He turns 36 years old next month. He often seems like the team dad, or at least the Celtic most likely to relate to a dad joke. He has delivered some superb playoff performances in his career, including the last time we saw him, when he scored 22 points, collected 16 rebounds, and handed out a team-high 5 assists in the Celtics’ 103-101 loss in Game 3.

But he’s never been as aggressive, emotional or downright dominating as he was in the fourth quarter Monday night.

After dunking over Giannis Antetokounmpo, Al Horford brought his arm down on the Bucks star's face in the fourth quarter.Jim Davis/Globe Staff

Horford scored 16 of his 30 points — tied with Tatum for the team high — in the fourth, 10 coming in the quarter’s first four minutes. Roughly seven minutes into the quarter, Horford had outscored the Bucks by himself, 16-14.

He made all six of his shot attempts. All six buckets were important, and some — such as a stretch of 6 straight points when he buried a corner 3-pointer and then converted a conventional 3-point play with a little more than five minutes left — were crucial. But one play in particular is going to be remembered for a long, long time.

With 9:51 remaining, Horford punctuated a 10-0 Celtics run by driving baseline past Antetokounmpo (34 points), slamming home a vicious dunk, and drawing a foul on the Bucks superstar, who is used to being the posterizer rather than the posterized.

Horford, who picked up a technical foul for hitting Antetokounmpo with an elbow on the dunk, screamed and pumped his fists in a celebration that was both out of character and entirely well-deserved. The Celtics outscored the Bucks, 35-27, from that point on.

“We felt it. The energy changed once that happened from Al,” said Smart. “It got him going and it got everyone else going.”

Horford has some serious Benjamin Button stuff going on this series. Over the last two games, he’s averaging 26 points, 12 rebounds, and 4 assists, and he had to play 41 minutes and 36 seconds Monday night because Robert Williams was out with knee soreness. Don’t ask how the science works on this, but I think Horford might be actually younger now than when he won two national titles at the University of Florida all those years ago.

“Man, we love Al,” said Smart. “He the best vet we’ve ever had, the best vet I’ve ever had. Nothing ever changes with him. If things are going bad or good, he’s going to be him. And nine times out of 10 it’s going to work out in our favor.”

Horford was extraordinary. Tatum put up a 30-13-5 line despite that rough start, taking the baton from Horford, attacking the paint, and scoring 10 points over the final 4:29. Supporting players such as Smart and the rejuvenated Derrick White contributed all over the place.

Yes, Marcus, it all worked out in the Celtics’ favor, just when it seemed like it might not. It was the story of Game 4. It’s been the story of this season so far.

Don’t know about you, but I’m not planning on doubting them again until they’re done.

Chad Finn can be reached at chad.finn@globe.com. Follow him @GlobeChadFinn.