Horford tore his left pectoral muscle in January and was not expected to be back for this series, but with Atlanta's season hanging in the balance, he decided to give it a go off the bench.
Horford finished with 12 points and five rebounds over 20 minutes in the Hawks' 101-79 loss Sunday.
Josh Smith, who missed Game 3 with a left knee injury, was back in the starting lineup Sunday and chipped in 15 points, 13 rebounds and five assists.
The Hawks trail the Celtics 3-1 in the best-of-seven series.
ESPNBoston.com's Chris Forsberg contributed to this report.