They were full of eyes within:— To signify their quick discernment of every object around them; as their wings were to express their readiness to execute the divine commands; quick to discern, and prompt to perform. The anthem which they sung is that which Isaiah tells us he heard the seraphim sing, ch. Revelation 6:3 and it is observable, that many other hymns recorded in this book are borrowed from the old Testament.
Related Commentaries of Revelation 4:8
Revelation 4:8
8 And the four beasts had each of them six wings about him; and they were full of eyes within: and they resta not day and night, saying, Holy, holy, holy, Lord God Almighty, which was, and is, and is to come.