Howard finished with 27 points, 14 rebounds and again made his free throws — 7 of 9 — for the resilient Magic, who made a team record 17 3-pointers — 11 after halftime.
Orlando can earn its first trip to the NBA finals since 1995 with a win in Game 5 on Thursday night in Cleveland.
Rafer Alston added 26 points and Rashard Lewis and Mickael Pietrus had 17 each for the Magic.
James was spectacular again, adding 12 rebounds and seven assists, but he had eight turnovers for the Cavs, whose season of seasons is slipping away.
"I had to get myself going. My teammates found me in the right place to score and I kept scoring," Howard said of his overtime effort that included three dunks, a tip-in and two free throws. "I hate losing, especially on my home floor. We kept fighting for the win."