John himself says that this is the voice he initially heard speaking to him like a trumpet. That’s the voice of Revelation 1:10, Revelation 1:12. We are not told explicitly who generated that voice (was it God? was it a mighty angel?) and so need to leave that question unanswered. What is important is that the same one (he’d spoken “like a trumpet” = loud and with authority) who first came to John with the instruction of Revelation 1:11 now comes with a second instruction. There is continuity between Revelation 1:1 – 3:22 and Revelation 4:1–11.
1 After this I looked, and behold, a door standing open in heaven! And the first voice, which I had heard speaking to me like a trumpet, said, “Come up here, and I will show you what must take place after this.”