Albert Barnes Commentary 1 Chronicles 9:26

"for the four chief porters, who were Levites, were in an office of trust, and were over the chambers and over the treasuries in the house of God." — 1 Chronicles 9:26 (ASV)

Rather, For the four chief porters, who were themselves Levites, were in trust, who also had the charge of the chambers, etc. A contrast seems intended between the four chief porters, whose charge was constant, and the remainder, who kept watch by turns.