The Lamb’s worthiness to take the book is triple (Revelation 5:9-10). He is worthy because:
1) He was slain
2) He purchased with His blood
3) He made the purchased-ones a kingdom and priests
The second and third of these meritorious acts, though, move out into a four. He purchased a people from every tribe, tongue, people, and nation. And the declaration that Jesus made the purchased-ones a kingdom and priests is followed by the final clause of the song: “they will reign on the earth.”
As elsewhere in Revelation, the triadic structure of the song hints at Trinity: Jesus the Son is the revelation of the whole Trinity; His sacrifice, purchase, and glorification is a Triune act.
Four is the number of earth. There are four winds, four points on the compass, four corners of the earth. Jesus’ threefold action gathers a people from the four corners, and His Triune work is completed only by a “fourth act” - the purchased ones reigning on earth as priest-kings.
We might want to add that 3 x 4 = 12, and 4 + 3 = seven. But that may be regarded as a step too far.