Zechariah 6 is the end of the vision that began in chapter 1. And, it also speaks again about the four horses, but it adds a something new to their description.
They are the Four Spirits that go forth from the Lord. And, they sound a lot like the Four Horsemen of the Apocalypse.
Chapter 6 ends with a crown for Joshua the High Priest, and how this six chapter prophecy will be confirmed. Even more important, this prophecy speaks of Jesus coming and building the Third Temple – the Body of Christ.