Atlanta’s Josh Smith takes the ball to the hoop as three Celtics converge on him during the second half of Saturday night’s Game Three.

The Journal / Glenn Osmundson

WALTHAM, Mass. — Atlanta shot 38 percent in the first two games of this first-round playoff series with the Celtics and lost. The Hawks shot 47 percent in the next two games and won.

The difference? Boston’s slip in defense in Games Three and Four.

The Celtics ran to a 16-3 lead in the first quarter Monday night but let the Hawks in the game by getting into foul trouble. Kevin Garnett, Paul Pierce and Rajon Rondo each picked up two in the first quarter and went to the bench.

The Celtics led by 10 at the start of the fourth quarter and by four late in the game but failed to make a stop when most needed and again allowed Atlanta to take the lead. Twice down the stretch the Hawks grabbed key offensive rebounds.

“The ball bounces off, and they get it,” Garnett said yesterday after a review of the game tape. “We needed a stop, and we would play defense for 22 or 23 seconds and the shot would go up and (Josh) Childress got a big rebound and (Al) Horford got another one. When you play defense you got to finish it off with rebounds. We didn’t finish off our defense.”

The Celtics didn’t have an answer to Hawks guard Joe Johnson in the two games at Philips Arena. He scored 23 points in Atlanta’s 102-93 victory in Game Three and 35 in the 97-92 triumph in Game Four. Johnson buried 14 of his 24 field-goal attempts in Game Four.

“When he’s in a zone like that, there’s not too many people who can stop him,” Kendrick Perkins said, adding that Johnson fed off the energy of the Atlanta crowd.

“Before we went to Atlanta, Joe hit one jump shot. We get to Atlanta and he hits jump shots and threes and elbow jump shots,” Perkins said.

So how do the Celtics stop him?

“We got to make him play in a crowd,” Perkins said. That means tighter defense on the perimeter and contesting layups when he drives to the hoop.

The Celtics must also execute better tonight.

“We could have trapped earlier,” Rivers said. “That’s something we don’t usually do because we don’t like to do it, but we had trap opportunities from our normal defense that we didn’t do. That’s what I mean by execution. There’s probably eight of his 20 that he should have never gotten the ball, never gotten the shot off without us making any adjustments. We saw that on film. A couple of times we were hugged up on guys instead of being in a spot of help. … Give him credit. He made some tough shots. We helped.”

Rivers was disappointed the Celtics tried to win with their offense instead of their defense.

“We had a 10-point lead going into the fourth quarter. … We’ve had 10-point leads all year, and you hit a stretch where you miss wide-open shots and then you go down the other end and you get a stop,” he said.

That didn’t happen Monday night.

“We were hoping to make a shot instead of hoping to make them miss a shot,” Rivers said. “We got to get back to the miss a shot mentality. … We got to do what we’ve done, better.”

Pierce expects the Celtics to play better in Game Five.

“Especially after Game Four, I saw a lot of positive things. Besides the fact that we lost, I thought we got a lot of good looks that we’ll take,” he said.

The Celtics aren’t surprised the Hawks tied the series on their home floor.

“I told the guys yesterday that in the second round in Seattle we took San Antonio to Game Six. We got blown out in their building two games and then two game back in Seattle we won those two and ended up taking it back to San Antonio,” said former Sonic Ray Allen.

“You can’t pre-determine how a series is going to go based on team records because it’s the playoffs. It’s a clean slate. You got to adjust to what teams are doing. You can’t be too arrogant to think teams are going to lay down for you.”

The Hawks certainly are not going to lie down for the Celtics, but they still have to win a game in the Garden. Their best chance may be tonight in Game Five, especially if the Celtics fail to play better defense.

mszostak@projo.com