Albert Barnes Commentary 2 Kings 21:4

"And he built altars in the house of Jehovah, whereof Jehovah said, In Jerusalem will I put my name." — 2 Kings 21:4 (ASV)

The “altars” of this verse seem to be the same as those of 2 Kings 21:5, and consequently were not in the temple building, but in the outer and inner courts.