Most of the first 3 chapters of Revelation (John’s Apocalypse) are later interpolation. The blatant Roman Asia references (Ephesus, Smyrna, etc) reveal much about who the scribe was and where they insisted John lived; this makes the beloved Saint Polycarp of Smyrna a candidate for the interpolation, assuming Polycarp even existed. Alternatively, the author of the later elaborate prologue might even have been Saint Irenaeus of Lyon.
This means that everything from 4:1 onward are authentic, except for some of the more obvious interpolations, such as Revelation 12:17 and Revelation 14:12 [Rationale: I don’t believe there were explicit references to Jesus in the original Revelation].
The first words of chapter 4 give indication that there was some prologue prior to the authentic part:
After this I looked, and there before me was a door standing open in heaven
The above would certainly be an awkward introduction, considering the phrase “After this” is a transitional phrase, indicating something preceded John’s encounter with heaven’s door.
I think the entire original prologue can be found in the first chapter of Revelation. Below is my reconstruction of what I think the earliest Revelation prologue was (using NIV translation):
[I was brought to the third heaven and ] I heard behind me a loud voice like a trumpet
I turned around to see the voice that was speaking to me. And when I turned I saw seven golden lampstands, and among the lampstands was a man dressed in a robe reaching down to his feet and with a golden sash around his chest. The hair on his head was white like wool, as white as snow, and his eyes were like blazing fire. His feet were like bronze glowing in a furnace, and his voice was like the sound of rushing waters. In his right hand he held seven stars, and coming out of his mouth was a sharp, double-edged sword. His face was like the sun shining in all its brilliance.
When I saw him, I fell at his feet as though dead. Then he placed his right hand on me and said: “Do not be afraid. I hold the keys of death and Hades.”
After this I looked, and there before me was a door standing open in heaven.