Charles Ellicott Commentary Zechariah 4:7

"Who art thou, O great mountain? before Zerubbabel [thou shalt become] a plain; and he shall bring forth the top stone with shoutings of Grace, grace, unto it." — Zechariah 4:7 (ASV)

O great mountain? — This is figurative of the colossal difficulties put in the way of the completion of the building of the Temple by the neighbouring powers. (Compare to Matthew 21:21.)

Thou shalt become a plain. — This certainly gives the true meaning of the original, which, however, is singularly graphic, and consists of but one word—literally, to a plain: that is, you shall become. The Septuagint mistakes the word for an Aramaic infinitive, and renders τοῦ κατορθῶσαι, “that you should bring it to a successful issue.” In the preceding vision, Joshua, as the high priest—in this, Zerubbabel, as the Prince of Judah—is the representative of the nation; in Zechariah 4:14, the two are referred to simultaneously.

Grace, grace unto it. — that is, to the headstone which, being the crowning stone of the building, is used to represent the whole Temple. The words are a prayer, which takes the form of a shout of triumph (like Hosanna!), and mean, May God’s grace or favour rest on the house forever!