Revelation 4 - The twenty-four elders

At which place is the description of Revelation 4?

Verse 1 starts that John has to come up into HEAVEN and look. So, the apostle John is NOW in HEAVEN and seeing the 24 elders. The 24 elders represent the true believers in Jesus Christ who accepted Him as Savior and Lord through FAITH. Therefore it INCLUDES ALL Old-Testament believers through FAITH!
Verse 2 at once John was in the spirit in Heaven. Like I once said, no human body can enter Heaven WITHA SINFUL BODY. Therefore ONLY after the Rapture of the Church and AFTER ONE has received a SINLESS BODY, one can enter Heaven. Thus HERE John enters Heaven in the spirit and not with his sinful body.

Who is He who sat on the throne in Revelation 4?

He who sat on the throne in Revelation 4 appears like jasper and carnelian, and round the throne was a rainbow that looked like an emerald. Carnelian differs from Jasper in that Jasper will always be opaque, whereas Carnelian is translucent. Carnelian may be nearly opaque but will still let light through when viewed from the edges. Jasper usually also exhibits multicolored patterns and splotches which are lacking in Carnelian.
The carnelian is a siliceous stone, ruby red and a species of chalcedony. Its color is a flesh-hued red, varying from the palest flesh-color to a deep blood-red. The carnelian stone represents as a stone of motivation and endurance, leadership and courage. It portrays God as our Redeemer.
Jasper represents as a stone of grounding and stability, providing comfort and security, strength and healing. It portrays God's absolute purity and perfection.
Greek sardion, a gem of a blood-red color. It was called "sardius" because obtained from Sardis in Lydia.
Israel's high priest wore the jasper and carnelian and ten other gemstones on his chest (Exodus 28:17–21). Each of the twelve stones represented a tribe of Israel. Just as the high priest wore the tribes of Israel symbolically on his chest, so Jesus, our High Priest carries us Christians on His heart. Jasper and carnelian are also the first and last gems, respectively, on the high priest's garments.
John also indicates that an emerald rainbow circled God's throne. An emerald is green and symbolizes life. The rainbow first appeared to Noah and his sons after the Flood as a token of God's promise to refrain from sending another flood to destroy the world (Genesis 9:8–17). The emerald rainbow around God's throne likely indicates that His promise of eternal life is guaranteed.

What is the meaning of the twenty-four elders and the four living creatures?

The meaning of the twenty-four elders in Revelation 4 is that the church members who are taken at the Rapture of the Church are in Heaven. They are saved from the Great Tribulation and are NO LONGER on earth. So Who are these twenty-four elders? They are the 12 elders who will judge the Jews and Israeiltes: the 12 tribes in Genesis 49 The sons of Leah: Reuben (Jacob's first-born), Simeon, Levi, Judah, Issachar and Zebulun; the sons of Rachel: Joseph and Benjamin; The sons of Bilhah (Rachel's handmaid) Dan and Naphtali; The sons of Zilpah (Leah's handmaid) Gad and Asher.
Revelation 7:4-8 And I heard the number of the sealed, a hundred and forty-four thousand sealed, out of every tribe of the sons of Israel, twelve thousand sealed out of the tribe of Judah, twelve thousand of the tribe of Reuben, twelve thousand of the tribe of Gad, twelve thousand of the tribe of Asher, twelve thousand of the tribe of Naphtali, twelve thousand of the tribe of Manasseh, twelve thousand of the tribe of Simeon, twelve thousand of the tribe of Levi, twelve thousand of the tribe of Issachar, twelve thousand of the tribe of Zebulun, twelve thousand of the tribe of Joseph, twelve thousand sealed out of the tribe of Benjamin. Pay close attention that in Revelation 7 the tribe DAN is replaced by the tribe Manasseh (first son of Joseph), one says because of his sin with Bilhad, his step-mother's maid and his father's concubine. Whether this also means that Dan will NOT judge the tribes, the Bible does not tell. I think that these 12 patriarchs of each tribe will judge all persons born out of his tribe. The other 12 of these 24 are the 12 apostles chosen by Jesus (whereby the traitor Judas is replaced by Paul), they will judge the Church, every believer in Christ. Because they received the gospel from Jesus, they taught the gospel and the New Testament. Therefore they know how hard it is to believe (they themselves asked to Jesus gives us more faith).
Clad in white garments, all believers in Heaven are cleansed through the blood of the Lord Jesus Christ, and therefore are blameless white, without sin and without a sinful body.

Is the church in Heaven in Revelation 4?

Verse 5 Before the throne burn seven torches of fire, which are the seven spirits of God; which represents the perfect (= number 7) Holy Spirit, so yes the Holy Spirit is with the Church in Heaven.

Verse 6 before the throne there is as it were a sea of glass, like crystal. And round the throne, on each side of the throne, are four living creatures, full of eyes in front and behind. Ezekiel 1:22 Over the heads of the living creatures there was the likeness of a firmament, shining like crystal, spread out above their heads. Verse 26 And above the firmament over their heads there was the likeness of a throne, in appearance like sapphire; and seated above the likeness of a throne was a likeness as it were of a human form.
A specific meaning commentaries are not able to assign to the sea of glass.

On each side of the throne, are four living creatures, full of eyes in front and behind: 7 the first living creature like a lion, the second living creature like an ox, the third living creature with the face of a man, and the fourth living creature like a flying eagle. 8 And the four living creatures, each of them with six wings, are full of eyes all round. See Ezekiel 10:14 And every one had four faces: the first face was the face of the cherub, and the second face was the face of a man, and the third the face of a lion, and the fourth the face of an eagle. Isaiah 6:1-3 I saw the Lord sitting upon a throne, high and lifted up; and his train filled the temple. Above him stood the seraphim; each had six wings: with two he covered his face, and with two he covered his feet, and with two he flew. And one called to another and said: "Holy, holy, holy is the LORD of hosts; the whole earth is full of his glory." So, these creatures have already been seen by Isaiah and Ezekiel.
The fullness of eyes represents ceaseless vigilance and unlimited intelligence, and their wings swiftness of movement. The four diffenent heads indicates wild beasts, domestically animals, human beings and flying creatures. The cherumbims give praise and adoration to its Creator and Savior and are used by God to execute His commands.
A perfect harmony between the believers and the cherubims (angels of God) adoring and obeying God the Father and the Lord Jesus Christ.

Verses 9-11 While the living creatures, angels, continue to sing praise, the 24 elders (= all believers in Heaven) fall down and ly their crowns before their Lord and join in the singing with the angels.