How do you explain Rome existing in John's time and yet it was not?
That phrase does not apply to the 4th kingdom at all. Why do people keep thinking Revelation was in the first century, as John's time? Preterism will never work. What was history cannot be explained as the final generation on earth. The final generation was not 1900 years ago.
John was a witness to the last 1,000 + years of current creation. John was not talking about Rome at all. Even those of the Reformation knew their current catholic pope as an AC was not the 4th kingdom. The RCC was not the same Roman government as the 4th beast. In Revelation 17, John said 5 kingdoms had fallen. Even the 5th kingdom no longer existed in "John's time". John was already experiencing today's generation in Revelation, since Revelation chapter 4. It was the 6th head and kingdom that was mortally wounded that was revived by Satan. Satan is the 7th head, but the 8th kingdom. Satan is the beast that was, and is not. You all should figure out why the 6th kingdom was in a state of death, not the 4th kingdom of the first century, of 1900 years ago.
The 5th kingdom died during the Reformation. There is hardly anything in Scripture about the 6th kingdom, thus no one talks about it. Daniel 2 only covers 5 kingdoms, that ended with the ten toe profile. Daniel never sees the 6th kingdom. No one in the first century saw the 5th kingdom. They were still living in the 4th kingdom. The 4th kingdom did not even end in 70AD. The 7th kingdom is not even part of this beast called a dragon. This beast has 7 heads, and there are 8 kingdoms mentioned. The 7th kingdom is not one of the heads. The 7th kingdom is related to the stone cut out without hands in Daniel 2.
"Forasmuch as thou sawest that the stone was cut out of the mountain without hands"
"And in the days of these kings shall the God of heaven set up a kingdom, which shall never be destroyed: and the kingdom shall not be left to other people, but it shall break in pieces and consume all these kingdoms, and it shall stand for ever."
Who is the King of this 7th kingdom? This kingdom has left the 6th kingdom in a state of death, never allowing the 6th kingdom to ever dominate the entire world like the first 4 kingdoms did. But in describing the 7th kingdom John said:
"and the other is not yet come; and when he cometh, he must continue a short space"
This is in contrast with the next thought:
"And the beast that was, and is not, even he is the eighth, and is of the seven, and goeth into perdition."
One can try to say the 8th kingdom immediately follows the 6th kingdom, but that is not how chronological progression works. In order for an 8th kingdom to be declared, the 7th kingdom has to be declared. The 7th kingdom was declared at the 7th Trumpet in Revelation 11:15. This 7th kingdom is still everlasting as it started at the Cross. But there is a point John makes that an 8th kingdom may also happen that will only last for 42 months. This is the 3.5 years of the AoD set up, and the ten horns come into play.
How can some posters explain 8 world kingdoms all happening prior to 70AD? Then some Amil claim the 7th kingdom has been the only kingdom since 30AD. Evidently the 5th and 6th kingdom never happened in their imagined interpretation. Although they acknowledge the 8th kingdom showing up just prior to the Second Coming, even if they don't call it the 8th kingdom.