come and see
The living one who introduces this second horse does not use a voice of thunder as did the living one who dispatched the first horse. This is significant, for it indicates that the first horse brings the other three horses in its train as a sequential consequence.1 See commentary on Revelation 6:1.
1 Monty S. Mills, Revelations: An Exegetical Study of the Revelation to John (Dallas, TX: 3E Ministries, 1987), Rev. 6:3.