Who are the four living creatures in Revelation?

The four living creatures are found in Revelation 4:6–9; 5:6–14;
6:1–8; 14:3; 15:7; and 19:4. The texts that describe these creatures
do not indicate that they are figurative—they are real, actual beings.
The four living creatures (literally "beings") are a special, exalted
order of angelic being or cherubim. This is clear by their close
proximity to the throne of God. Ezekiel 1:12–20 suggests that they are
in constant motion around the throne.

Revelation 5:6–14 describes the duties or purposes of the four living
creatures. They fall down and worship the Lamb, Jesus Christ, offering
the same reverence to Him that they did to the Father (Revelation
4:10), proof positive of the deity of Jesus Christ. Along with the 24
elders, they have "harps and golden vials full of incense, which are
the prayers of the saints." Harps are frequently associated with
worship in the Old Testament, as well as with prophecy (2 Kings 3:15;
1 Chronicles 25:1). Incense represents the prayers of the saints.
Therefore, taken together, the four living creatures and twenty-four
elders hold in their hands all that prophets ever prophesied and
believers ever prayed for—all about to come to pass.

The purpose of the four living creatures also has to do with declaring
the holiness of God and leading in worship and adoration of God, and
they are involved in some way with God's justice, for when He opens
the first four seals and sends out the four horsemen to destroy, their
powerful voices, like thunder, command "come" (6:1–8). The horsemen
respond to the summons of the four powerful creatures, indicating the
power the creatures possess. That power is seen again in Revelation
15:7 when one of the four unleashes the last seven plagues of God's
wrath on mankind.

The four living creatures are very similar, if not the same, beings as
those in Ezekiel 10:2, 14, and 20 and Isaiah 6:1–3. They are four in
number, full of eyes, have faces like the beings in Ezekiel 1:10, have
six wings (Isaiah 6:2), and offer worship as the beings in Isaiah 6:3,
saying, "Holy, holy, holy is the Lord." They may not be the exact same
beings, but they definitely are comparable and probably of the same
order.

In summary, these beings are an exalted order of angels whose purpose
is primarily that of worship (Revelation 19:4). They are very similar
to the beings in Ezekiel 10 and Isaiah 6:1–3, and they are in some
ways involved in God's divine justice.