Holy, Holy, Holy are sung by four living creatures as worship to the One upon the throne. In the present tense, and in the future, whatever these creatures are shall give glory to the Creator in triune praise.
As soon as the living creatures, or Cherubim sing, the twenty-four elders immediately fall down in worship to the God who is revealed through all of His creation.
The four Cherubim give glory, honor, and thanks to the King of the Universe, for eternity. The continual praise is because of His continual acts of sovereignty, as to always honor Him.
He is attributed with Lord God Almighty, or the Highest of Highest God who was from everlasting and is always present and is to come and reign in His glory.
8 And the four living creatures, each of them with six wings, are full of eyes all round and within, and day and night they never cease to sing, “Holy, holy, holy, is the Lord God Almighty, who was and is and is to come!” 9 And whenever the living creatures give glory and honor and thanks to him who is seated on the throne, who lives for ever and ever, 10 the twenty-four elders fall down before him who is seated on the throne and worship him who lives for ever and ever; they (i.e, the 24 elders) cast their crowns before the throne, singing, 11 “Worthy art thou, our Lord and God, to receive glory and honor and power, for thou didst create all things, and by thy will they existed and were created.” Rev. 4:8-11
The Cherubim, an angelic order of created beings, seems to always be about the throne. By implication, they frequently praise God, especially as it relates to unveiling justice throughout The Revelation. They are God’s agents who assist in opening the seals which bring about these judgements.
The four living creatures are similar beings as those found in Ezekiel and Isaiah. Full of eyes, they see everything and watch over the four corners of the earth. Their faces are like those in Ezekiel 1:10, they have six wings like those in Isaiah 6:2 and they praise God just like the seraphim in Isaiah 6:3.
Crowns cast by the 24 elders are the crowns of those who humbly submit to the King. These are they which overcame by the blood of the Lamb.
Matthew Henry may have said it best:
“4:9-11 All true believers wholly ascribe their redemption and conversion, their present privileges and future hopes, to the eternal and most holy God. Thus rise the for-ever harmonious, thankful songs of the redeemed in heaven. Would we on earth do like them, let our praises be constant, not interrupted; united, not divided; thankful, not cold and formal; humble, not self-confident”.