"After this I looked, and there before me was a door standing open in heaven. And the voice I had first heard speaking to me like a trumpet said, 'Come up here, and I will show you what must take place after this.'" NIV translation
John tells us of a "door standing open" which looks into heaven. We notice that John tells us that it is "a" door and not many doors. That should remind us that the only way to see heaven is through Jesus Christ. Although it is popular in the world today to say that there are many ways to heaven, the simple fact of the matter is that it can only happen through Jesus Christ.
The voice tells John that he will show him "what must take place after this". We are transitioning from the instructions and pleas to the church to the end times events.
Revelation 4:2 & 3
"At once I was in the Spirit, and there before me was a throne in heaven with someone sitting on it. And the one who sat there had the appearance of jasper and carnelian. A rainbow, resembling an emerald, encircled the throne." NIV translation
John was "in the Spirit" which is the only way that we enter into heaven. The first thing he saw was a throne which represents royalty and the one on the throne was as precious as jewels. The throne was encircled by a promise which is the rainbow. The rainbow was given to all men after the Great Flood as a symbol of the promise from God that he would not destroy the earth again with water. Now, it is fitting that John's first look into heaven reveals the fulfillment of the precious promise to be with the King Of Kings. That promise is for all who are willing to go through that open door that is Jesus Christ.
"Surrounding the throne were twenty-four other thrones, and seated on them were twenty-four elders. They were dressed in white and had crowns of gold on their heads." NIV translation
John continues to describe the scene of the throne in heaven. The twenty-four elders are the twelve leaders of the tribes of Israel and the twelve apostles. They are dressed in white as a symbol of purity and the crown is a symbol of victory.
"From the throne came flashes of lightning, rumblings and peals of thunder. Before the throne, seven lamps were blazing. These are the seven spirits of God." NIV translation
The description here is of power coming from the throne. Have you ever just sat and watched a thunderstorm? You can just feel the power in it and this is probably what John felt as he looked at the throne. We know that the number seven symbolizes completeness and here we see that the complete spirit of God is around the throne.
Revelation 4:6 & 7
"Also before the throne there was what looked like a sea of glass, clear as crystal. In the center, around the throne, were four living creatures, and they were covered with eyes, in front and in back. The first living creature was like a lion, the second was like an ox, the third had a face like a man, the fourth was like a flying eagle." NIV translation
John now sees four living creatures covered in eyes. These creatures are also described in the book of Ezekiel from his vision. We do not get much information about these creatures except that they continually praise God.
"Each of the four living creatures had six wings and was covered with eyes all around, even under his wings. Day and night they never stopped saying: 'Holy, holy, holy is the Lord God Almighty, who was, and is, and is to come.'" NIV translation
John now tells us how the creatures praise God. They are covered with eyes which symbolizes seeing everything. It brings to mind the fact that they see all of the glory of God and their reaction is to continually say how holy God is. For us, if we reflect on all that God is and has done, then we too may have the same reaction and see just how holy God is.
"Whenever the living creatures give glory, honor and thanks to him who sits on the throne and who lives for ever and ever, the twenty-four elders fall down before him who sits on the throne, and worship him who lives for ever and ever. They lay their crowns before the throne and say: 'You are worthy, our Lord and God, to receive glory and honor and power, for you created all things, and by your will they were created and have their being.'" NIV translation
When the living creatures worship, the elders get involved as well. They worship the creator with their mouths and cast their crowns before the throne. This is a pattern that we too shall follow when we get to heaven.