After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will show you things which must be hereafter. And immediately I was in the spirit: and, behold, a throne was set in heaven, and One sat on the throne. Revelation 4:1,2
Chapter 4 begins with an open door and a voice like a trumpet which says: "Come up here, and I will show you things which must take place after this". This is the second time John has heard the voice 'like that of a trumpet'; the first was in Revelation 1:10, when he was in the spirit on the Lord's Day. John was then given a very specific task; to serve as a witness and record future events: Rev 1:19.
Now, the voice as a trumpet speaks again, and John is taken immediately into the heavenly realm, bypassing spiritual dimensions to serve as a witness, recording the events shown to him. He was not given dreams and visions; instead, he becomes spirit, carried into the day of the Lord to see the things which will be.