Charles Ellicott Commentary 1 Chronicles 15:4

"And David gathered together the sons of Aaron, and the Levites:" — 1 Chronicles 15:4 (ASV)

And David assembled. — He confers separately with the priestly order respecting their part in the procession.

The children of Aaron. — The sons of Aaron, that is, the high priests, Zadok and Abiathar (1 Chronicles 15:11).

The Levites — that is, the six chieftains—Uriel, Asaiah, and Joel, heads of the clans of Kohath, Merari, and Gershom respectively; and Shemaiah, Eliel, and Amminadab, additional Kohathite chiefs. All six were at the head of their clansmen (brethren, 1 Chronicles 15:5–10).

There were four Kohathite houses to one of Merari and Gershom because the sub-tribe of Kohath was the elder house and had special charge of the Ark and other most holy vessels of the sanctuary (Numbers 4:4).