Ryan Leif Hansen (Silence and Praise) observes that the command to “Come” frames the book of Revelation. As the Lamb opens the first four seals, each of the four living creatures calls for a horseman to “Come” (6:1-8), and the book ends with three more commands. The Spirit and bride, and the one who hears the book, join in saying “Come” (2x in 22:17). Then an unnamed speaker (John?) ends with “Come, Lord Jesus.” Presumably the Spirit, bride, and hearer are also asking Jesus to come.

4 + 3. That equals 7. And makes a neat numerological frame.

And that might give us an important clue to a contested question: The identity of the horsemen in Revelation 6. Are they agents of God? Agents of hell, whom God permits to wreak destruction? If there's a pattern of seven calls to “Come” stretching from chapter 6 to 22, and if the latter three are calls for Jesus to come, then perhaps the first four are also calls for Jesus to come. 

The shift from cherubic (“living creature”) calls to human calls fits the overall sweep of Revelation, which moves from angelic and animal to human. At the outset, the cherubim call on riders to come, as the saints cry for the coming of vengeance. When we get to he end of the tribulations of the book, the voices of cherubim have yielded to the voice of the Spirit, who joins voices with the Bride and with the hearer of the book. No longer animal/cherubs, but Spirit-inspired human voices call for Jesus to come.

Many commentators think that the first horseman on a white horse is Jesus (cf. 19:11-16). But this structural thread suggests that the others are as well. Jesus rides the red horse bringing war; the black horse who destroys wheat and barley but preserves the oil and wine; the green horse, bearing the name Death. That suggestion would need to be unpacked to be persuasive, or plausible, but the pattern of “Come” points decidedly in this direction.