And the twenty-four ELDERS and the four LIVING CREATURES fell down and WORSHIPED GOD
The twenty-four ELDERS generally are believed to be representative of the redeemed believers in heaven. Neither the ELDERS nor the LIVING CREATURES are mentioned in Revelation after this. After the marriage of the redeemed believer (bride) to Jesus Christ, (the revealed One, bridegroom) takes place (verse 7), there is no need to distinguish between the two --- they have become ONE (Ephesians 5:22-31-33). From here on out, the Revelation truly is the Revelation of Jesus Christ.
On the other hand, the four LIVING CREATURES have positions around the THRONE of God. They continually acknowledge or WORSHIP God for the total and complete GOOD that God is and that is why they continually say "AMEN" (Revelation 5:14). The LIVING CREATURES, by being so close to God, appear to see everything (Revelation 5:8-10). Since they are full of eyes, front and back and within (Revelation 4:6,8), they fully understand what the complete destruction of Babylon (evil) means. It is as if they are to heaven as the tree of the knowledge of good and evil is to the earth (Genesis 2:15-17). Even as we, today, cannot find the tree of the knowledge of good and evil, so too in heaven after this verse, the four LIVING CREATURES are not heard from again. That is not to say that they are removed from their position around the THRONE.
who sits on the THRONE saying,
Please refer back to Revelation 4 for the details of this place in heaven.
"Amen. Hallelujah!"
When the word of God, whether as the Law (Nehemiah 8:1-12) or Salvation (Luke 15:3-7) or Judgment (this portion of Revelation), the response is or should be "AMEN" and "HALLELUJAH!". Israel's sins and punishment (Psalm 106) can be thought of as a microcosm of the whole earth's iniquities and punishment. When the final realization of what God did to save His chosen people, the people are to say "Amen. Praise the Lord!"