Revelation 4 starts off with a distinct Greek phrase: Мετα ταυτα (meta tauta), “after these things.”  It is used to indicate a transition from one Revelation event to another.  In the first three chapters John had a vision where the seven churches were examined by Jesus and warned of future events and how to prepare.  Then everything changed, the focus and direction changed: Мετα ταυτα ειδον (meta tauta eidon) “After this I saw…” 

In Revelation 4:1 John turns his attention in another direction, he sees an opened door in heaven, and he hears a voice like a trumpet, telling him, “Come up here, and I will show you things…”  John’s orientation change is so profound that he is transported in the spirit to a throne room in heaven. 

This “voice like a trumpet” should not be confused with the seven Trumpets of Judgment; it is just a figure of speech meaning “loud and clear.”  Consider a previous verse in the same book, Rev 1:10-11, “…and I heard behind me a loud voice like a trumpet, which said: “Write on a scroll…” or Isaiah 58:1, “Shout it aloud, do not hold back.  Raise your voice like a trumpet.  Declare to my people their rebellion and to the house of Jacob their sins.”

People, who believe in the pre-Tribulation rapture, say Revelation 4:1 means, “After the Church was gone, I saw…” but the Church is not stated or even implied in the transition or in this verse.   It is simply a transition.