What do the four living creatures never stop saying, day and night?

Answer: "Holy, Holy, Holy"

They never stop saying, "Holy, Holy, Holy, is the Lord God Almighty, who was and is and is to come!"

“And each of the four living creatures had six wings and was covered with eyes all around and within. Day and night they never stop saying: “Holy, Holy, Holy, is the Lord God Almighty, who was and is and is to come!””— Revelation 4:8 (BSB)