The elders are differentiated from the angels (cf. Rev. Rev. 5:11+). They are either a separate class of angelic beings (much like the living creatures), or they are redeemed men. See commentary on Revelation 4:4.
the four living creatures
See commentary on Revelation 4:6.
προσεκύνησαν [prosekynēsan] : Used to designate the custom of prostrating oneself before a person and kissing his feet, the hem of his garment, the ground, etc.; the Persians did this in the presence of their deified king, and the Greeks before a divinity.1 The frequent response of the elders and living creatures (Rev. Rev. 4:10+; Rev. 5:14+; Rev. 11:16+; Rev. 19:4+). An innumerable host worships before the throne (Rev. Rev. 5:11+).
1 Frederick William Danker and Walter Bauer, A Greek-English Lexicon of the New Testament and Other Early Christian Literature (Chicago, IL: Uersity of Chicago Press, 2000), 716.