Mario Chalmers was struggling his way through the NBA finals. That is, until the Miami Heat needed him most.

So in the biggest game -- and biggest moments -- of the season, he delivered.

Chalmers scored 19 of his 25 points in the second half, 12 of them coming in the fourth quarter, as the host Heat moved one win away from a championship with a 104-98 win over the Oklahoma City Thunder in Game 4. The Heat leads the series 3-1 and can clinch the crown with a home victory Thursday night.

"No matter what, no matter how tough we are on him, he actually thinks he's the best player on this team, and that's a gift and a curse," Heat guard Dwyane Wade said. "But tonight it was a gift for us because he never gets down on himself, he always believes, 'Find me, I can make a shot. I can make a play.' He was huge for us."

Chalmers is the official Heat locker room criticism lightning rod. LeBron James, in particular, thinks nothing of screaming a few things at Chalmers after he makes a mistake in a game. Because Chalmers usually seems undeterred by it all, the Heat has never seen a reason to stop.

"We've been staying on him because we need him," Heat forward Chris Bosh said. "We knew coming into the season we needed him to be a better point guard. He's just outlasted a tremendous amount of pressure, and he's responding every time."

Thunder blunder: Russell Westbrook delivered a masterpiece, zigzagging around flummoxed Heat defenders and scoring 43 points in 45 minutes. But he made a regrettable mistake late in the game, and his brilliance ultimately went to waste.

After Westbrook's bank shot pulled the Thunder within three with 41 seconds remaining, he foolishly fouled Chalmers after the Heat won a jump ball with 14 seconds to go but with just four on the shot clock.

Westbrook didn't know the shot clock had not been reset. Chalmers hit both free throws, stretching Miami's lead to five. "Miscommunication on my part," Westbrook said.

But Thunder coach Scott Brooks said "one play does not determine the outcome. Russell was terrific; the guy played relentless. He kept us in this game."

Kevin Durant, who shot just 2 for 6 in the fourth quarter of Game 3, attempted only three shots in the fourth quarter of Game 4, making two of them.

"They're sitting guys in my lap," Durant said of the Heat's defense. "Sometimes they're doubling. I'll find a way to score."

Lakers: Ramon Sessions is declining his $4.55 million player option for next season and will test free agency. The Lakers said they still will try to sign the point guard.

The Miami Herald contributed to this report.