Revelation 4-5 describe the throne of God. God has a Kingdom, and hence, it is the “Kingdom of God”. It is the only Kingdom of the heavens, hence, it is the “Kingdom of Heaven”. Psalm 145:13 says it is eternal, or, everlasting.
God chooses to be a king and to have a Kingdom, or rather, it is His nature. He reigns by virtue of who He is. In that place, though, He chooses to let us rule and reign with Him, under Him, with a portion of His authority.
In this section, John sees the heavenly throne-room of God, as well as what goes on there. This is a heavenly vision, and the scripture relates both the dynamics of receiving that vision as well as the throne room and the events there. So, we have three things recorded. We have the mechanics of the vision, as it were, of heavenly doors being opened and John going to the throne room. Second, we have the throne room scene itself, which corresponds to the other records of this place in the Scripture. Finally, there is what is going on there at that particular time. This is similar to if two people visited the throne room of a king or queen, say of England, on two separate occasions. There is both how they came to the place, whether this door or that, or who escorted them and why, and then there is the relating of the scene. The general surroundings will always be same, but the events of the throne are what constitute the body of the vision.