Charles Ellicott Commentary Zechariah 4:9

"The hands of Zerubbabel have laid the foundation of this house; his hands shall also finish it; and thou shalt know that Jehovah of hosts hath sent me unto you." — Zechariah 4:9 (ASV)

Thou ... unto you. —Such a change in number is common in Hebrew, especially when addressing a nation, which at one time is looked on as a corporate unity, at another as a collection of individuals. Or “thou” may have been addressed to Zerubbabel, and “you” to the people, when the prophet delivered his oracle to them.