Jamieson, Fausset and Brown on Revelation 4:9-11

9-11. The ground of praise here is God's eternity, and God's power and glory manifested in the creation of all things for His pleasure. Creation is the foundation of all God's other acts of power, wisdom, and love, and therefore forms the first theme of His creatures' thanksgivings. The four living creatures take the lead of the twenty-four elders, both in this anthem, and in that new song which follows on the ground of their redemption (Re 5:8-10).

when--that is, whensoever: as often as. A simultaneous giving of glory on the part of the beasts, and on the part of the elders.

give--"shall give" in one oldest manuscript.

for ever and ever--Greek, "unto the ages of the ages."