Bible Notes Online - Zechariah 6 - ESV
Commentary

v1-8: The vision of four chariots, identified as the four spirits of Heaven, who go from the presence of the Lord into the whole earth. The chariot with the black horses gives rest to God's Spirit in the land of the north, as in 1.11,15. In chapter 1, the rebellion in the earth is judged, bringing peace; here God's Spirit is at rest, for God's work of judgment has been accomplished.

v9-11: The crown for Joshua, the high priest; a foretaste of Christ, who is both priest and king. Joshua's father, Jehozadak is referred to in 1 Chr 6.15, in the line of Zadok, for whom a blessing is promised, Ezek 44.15-16.

v12-13: The Branch, Jer 23.5-6, the priest sitting as king; Christ of the order of Melchizedek and the line of David; both endless dynasties are fulfilled in Christ; priest forever (Heb 7.17, quoting Ps 110.4) and king forever (2 Sam 7.16).

v14: The crown is laid up before the Lord, for Joshua is not the fulfilment of the prophecy, but merely a shadow of the glorious priest-king to come.

v15: The rebuilding of the temple is a specific sign from God, as a blessing upon the people who returned with Zerubbabel, and a vindication of Joshua and Zerubbabel. There is however an on going challenge to obey.