There are several references to angels during the time of the Tribulation, which means that they have a prominent role and responsibility in all that God sets forth to do during the great time of His wrath.
After the letters to the seven churches (Rev. 2-3), John, in a vision saw through an open-door, certain things happening in heaven. He saw One (God) sitting on a throne encircled by a rainbow. And there were twenty-four thrones around God’s throne, and twenty-four elders were seated on them, having on white garments with gold crowns on their heads. And there were other strange and glorious things that John saw, including the presence of four living creatures.
Four living creatures (Rev. 4:6-9). These four living creatures it seems were placed closest to God and His throne, and so they were His guardians. The sight of them were as such: they were “full of eyes in front and behind.” One (or the first) creature was like a lion, and the second like a calf, and the third had a face like a man, and the fourth was like a flying eagle. And they all had six wings. John MacArthur writes,
These are the cherubim, those angels frequently referred to in the Old Testament in connection with God’s presence, power, and holiness.
The testimony that John gave of them was this: day and night they did not cease to say,
HOLY, HOLY, HOLY is THE LORD GOD, THE ALMIGHTY, WHO WAS AND WHO IS AND WHO IS TO COME.
And John saw that when these angels would give glory and honor to God, the twenty-four elders would fall down and worship Him. Hence, one of the purposes of these glorious ones were to lead and inspire worship in the elders (see also Rev. 19:4).
Besides being leaders in worship they were involved in the administration of the various judgments, as we see in Revelation 15:7, where one of them gave to seven angels, seven golden bowls full of the wrath of God (see also Rev. 5:11, 14; 6:1, 3, 5, 7; 7:11; 14:3; 19:4).
A strong angel (Rev. 5:2). We don’t know for sure, but this may be a reference to the angel Gabriel, because his name means “strength of God.” Anyway, his particular role was to ask the question, “Who is worthy to open the book and to break its seals?”
“The book” was held by God on His throne and it was sealed up with seven seals (v. 1). No doubt the book is the title deed to the earth and it describes seven things God will do to the earth.
Now as to the question that the “strong angel proposed, no one seemed to have an answer, and John began to weep because no one was found worthy to open the book.
But then, out of the blue, one of the elders said, “Stop weeping! Look over there.” And he pointed out a small pet lamb that had been slain (and killed), but was now alive and standing. And the elder said of Him that He was “the Lion that is from the tribe of Judah, the Root of David [and He has] overcome so as to open the book and its seven seals.”
So we see two things here: 1) We see in the lamb the true Passover Lamb, which is God’s Son. He was slain for our sins, but He is now alive. He has been resurrected and has triumphed over sin and Satan. 2) We also see that He is called the “Lion from the tribe of Judah, the Root of David.” This is clear confirmation that He is the living Messiah, the descendant of David. And so, the Lamb (which is also the Lion of the tribe of Judah) are found worthy to open the book and its seven seals. And so, having this confirmation, the Lamb came and took the book out of the right hand of God.
And then, in a response of praise to the lamb and to God, the four living creatures and the twenty-four elders fell down before the Lamb and they sang a new song, saying,
Worthy art Thou, our Lord and our God, to receive glory and honor and power; for Thou didst create all things, and because of Thy will they existed, and were created.
Many angels (Rev. 5:11). And then besides the voices of the elders and the living creatures, John heard the voice of many angels numbering “myriads of myriads and thousands of thousands—which could be translated as innumerable or as an amount beyond calculation. And they said with a loud voice,
Worthy is the Lamb that was slain to receive power and riches and wisdom and might and honor and glory and blessing.
And besides these many angels, the entire universe began to praise both God and the Lamb (v. 13).
Four angels, and another angel (Rev. 7:1-3). After the first six seal judgments and just before the seventh seal judgment is an interlude of time (Rev. 7:1-17); and to begin that period of time, four angels appeared and stood at the four corners of the earth—that is, north, south, east, and west. And it was their job to hold back the wind from all directions. And then John saw another angel holding a seal of God in his hand. And he cried out to the four angels saying, “Do not harm the earth or the sea until the bond-servants of God (the 144,000 Jews) are sealed on their foreheads.” The seal was apparently a mark put on to protect them from harm; the harm that was soon coming to the earth by the same four angels.
All the angels (Rev. 7:11). After the 144,000 Jews were sealed, suddenly there erupted loud praises in heaven around the throne of God. It first came from the great multitude of resurrected martyrs (Rev. 7:9-11; 13-16). And then, all the angels (probably the same group of angels in Rev. 5:11), plus the elders and the four living creatures joined in the praise. And then all of them fell on their faces and said,
Amen, blessing and glory and wisdom and thanksgiving and honor and power and might, be to our God forever and ever. Amen.
I think, apparently, the praise was to God for His salvation of all those martyrs who were saved and now enjoying eternal life (Rev. 7:15-16).
 John MacArthur, Ibid, John MacArthur’s notes of Revelation 4:6.