Revelation 4 is not the first time we meet the living creatures, but we do see them in Ezekiel 1 (and perhaps Isaiah 6). John tells us briefly of their appearance, “The first living creature was like a lion, the second living creature like a calf, the third living creature had a face like a man, and the fourth living creature was like a flying eagle” (Revelation 4:7).
I believe these represent the living narrators of Jesus on the earth, the Four Gospels, Matthew, Mark, Luke, and John, represented in Revelation 4 in that order. Read Revelation 4 to see what they do.