The scene in heaven in Revelation 4 is a temple scene. It’s also the beginning of John’s sky journey among the stars and constellations. But the combination of a throne and thrones indicates another setting: This is a courtroom.
That’s what thrones indicate elsewhere in Scripture (Psalm 122:5; Daniel 7:9), most especially in Revelation 20:4, to which I will return momentarily.
If this is a meeting of a court, it is an abortive assembly. No case is presented. The officers of the course seem to abandon the process in mid-stream, throwing their crowns down to the One Enthroned, as if He alone were presiding.
No one, after all, can open the book (5:4), and if a court is going to make a decision it needs the evidence that the book provides (cf. Daniel 7:9).
The remainder of Revelation is the preparation for a reconvening of the court. The Lamb appears, opens the book, presents the evidence. He’s worthy to take the book because He gave Himself to constitute a people as priests and kings. The Lamb is the judge, but He is so because He died to assemble a court of the saints. When all has been opened, John sees thrones set up again (20:4) and then books are opened so that the living and the dead can be judged (20:12).
Insofar as it takes place in a courtroom, Revelation is a forensic drama. It’s about justification.
For a somewhat different development of the juridical themes of the book, see Alan Bandy, The Prophetic Lawsuit in the Book of Revelation.