According to Trinitarians, who is seated on the Throne and who takes His scroll from His right hand, in Rev 4 and 5?

According to Trinitarian theology, there is one God. This God is the one on the throne - He is Father, Son and Spirit. This one God is called Yahweh.

We must be careful to not take Revelations too seriously/literally as this will cause unnecessary contradictions.

The Lamb, the one taking the scroll, is the ascended Jesus - the human nature of the eternally begotten Son who co-created with the Father. While this may sound a bit obscure, these are figurative visions that John is describing and not intended to be an actual, literal account, of what is happening.

Of course, God is invisible, whereas Jesus is not. He remains visible being the human derivative of the Son becoming flesh through the Word. The ascended Jesus who 'sits at God's right hand' (and takes the scroll) is also a figure of speech not intended to be taken literally as there are not two sons! John is having difficulty describing these events and trying to put into words the nature of God and the human incarnation of God in the flesh.

Putting all these verses together, one might conclude that it is YHWH who creates all things and sits on the throne, and the Lamb is someone else.

Yes, that is what may seem to be the case if one takes the verses mentioned literally instead of figurately as required by Trinitarian dogma which is a mystery not to be fully grasped in this age.

OP: Putting all these verses together, one might conclude that it is YHWH who creates all things and sits on the throne, and the Lamb is someone else.

But is this what Trinitarians think?

Not exactly, from a Trinitarian understanding there is the obvious sense that God the Father may be on the throne and the Lamb is God the Son.

OP: According to Trinitarian interpretations of Revelation 4 & 5, who is being worshiped as the Lord God Almighty who created the heavens and earth? And who is this lamb who takes the scroll from the Creator's right hand?

If we read on to the end of chapter 5, it is clear both are worshipped.

Saying with a loud voice, Worthy is the Lamb that was slain to receive power, and riches, and wisdom, and strength, and honour, and glory, and blessing. And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them, heard I saying, Blessing, and honour, and glory, and power, be unto him that sitteth upon the throne, and unto the Lamb for ever and ever. And the four beasts said, Amen. And the four and twenty elders fell down and worshipped him that liveth for ever and ever. Rev 5:12-14 (emphasis mine)

Here we see worship given to the One on the throne and to the Lamb for ever and ever. They fall on their faces and worship the Lamb who lives for ever and ever.

As to the other comments about YHWH being said to be solely God Almighty, we also find this.

Hast thou not known? hast thou not heard, that the everlasting God, the LORD, the Creator of the ends of the earth, fainteth not, neither is weary? there is no searching of his understanding. Isa 40:28

Thus saith the LORD, thy redeemer, and he that formed thee from the womb, I am the LORD that maketh all things; that stretcheth forth the heavens alone; that spreadeth abroad the earth by myself; Isa 44:24

Note the definition of LORD, the redeemer.

Christ hath redeemed us from the curse of the law, being made a curse for us: for it is written, Cursed is every one that hangeth on a tree: Gal 3:13

So, Trinitarians understand God the Father, Son, Spirit. The one who created is God, the one being worshipped is God.

The one seated on the throne holding the scroll is the Father. The description is of Him who lives forever and ever. To him alone belongs immortality.

The one who takes the scroll is Jesus, the Lamb of God.

The scroll is the Plan of Salvation. Though God had a plan to save mankind, nobody was able to put that plan into effect until the penalty of sin had been paid.

Only He that has the keys of Hades and of Death can open that scroll, can unseal the plan of salvation and put it in motion. Without the sacrifice of Jesus, no one in heaven or on earth or anyone anywhere could do anything to save man.

The whole plan of salvation, like the Bible, revolves around the sacrifice of Jesus. It is central to the issue.

There is a crucial difference between chapter 4 of the Revelation, and chapter 5. Here I quote from a Trinitarian Christian in his book, explaining the significance of why the Lamb is not mentioned in relation to the throne of God until chapter 5. This will help answer the question. The quote is dealing first with the matter of how Abraham and his Seed (which is Jesus Christ - Matthew 1:1 & Galatians 3:16, and those who then are found 'in Christ,' belonging to him vs. 29) inherit the world, Romans 4:13.

"The question was, How should Christ - with his redeemed people - become heir of the world, actually occupying the throne of world dominion? That was the sealed book.

Now consider the throne depicted in Chapter 4. It was not the throne of grace. Christ was not yet seen to be upon that throne, compare Rev. 4:2,3 with 5:7,13. It was not the throne of judgment. That appears in Rev. 20:11,12. But it is the throne of glory. Heavenly glory. And he is seen in a context which shows indisputably the assertion of his rights over Creation. It is his Creation. But then there is this book. It is in the right hand of him who sat upon the throne. This throne is that of the Majesty in the heavens.

This is that Majesty who reigns absolutely and sovereignly over all the kings and rulers of the earth. They know nothing of this. But it is his earth, not theirs.

Listen to this: 'Thou art worthy, O Lord, to receive glory and honour and power: for thou hast created all things, and for thy pleasure they are and were created', Rev. 4:11. And again, hear the redeemed cry to the Lamb in heaven, Rev. 5:10, 'And hast made us unto our God kings and priests; and we shall reign on the earth'. Likewise observe Rev. 5:13, 'And every creature which is in heaven, and on the earth, and under the earth, and such as are in the sea, and all that are in them, heard I saying, Blessing and honour, and glory and power', a fourfold. earthly, eulogy - 'be unto him that sitteth upon the throne, and unto the Lamb for ever and ever.'

If this be not all Creation, what is all Creation? ...why then do ten thousand times ten thousand angels say with a loud voice, 'Worthy is the Lamb that was slain to receive power, and riches, and wisdom, and strength, and honour, and glory, and blessing', a sevenfold doxology to him who had these things in heaven, but looked for them on earth, Rev. 5:11,12." The Revelation of Jesus Christ, pp.111-112 http://www.johnmetcalfepublishingtrust.co.uk/contact_us.htm

The point is that whatever anybody thinks about who is seated on this throne of glory, and who the Lamb is who later comes to take the book out of God's hand, all Creation will fall down in worship before him who sits on the throne and the Lamb. Even the enemies of God and Christ will, one day, fall down in worship and confession, prior to being sent to their eternal destiny.

The reason why the Lamb does not appear in the center of that throne of glory until chapter 5 is that the Revelation is unfolding events, and there could be no Lamb that appeared as had been slain, until after that slaying, and after the resurrection of that Lamb of God, who takes away the sin of the world. Chapter 4 gives us a glimpse into heaven prior to that event on earth, and chapter 5 shows the difference in heaven thereafter. The Lamb is standing in the center of that throne of glory, all heaven worshipping him, and its other occupant - God.

The elders, who are also said to be in the midst of the throne, never receive worship, but give their worship to God and the Lamb, on that throne (Rev. 5:8,9).

Only trinitarians can make sense of that. Non-trinitarians remain perplexed, and nothing a trinitarian says will make the slightest difference, because it requires divine revelation to have one's eyes opened as to just who this Christ - this Lamb - actually is. And that is a work of the Holy Spirit. But those who keep reading the Holy Spirit inspired scriptures, sincerely seeking to learn and to be taught by Him, may have that glorious revelation revealed to them.

In Summary: It is Almighty God, the Creator, who sits on that throne of glory, that book in his right hand. No-one in all Creation can open that sealed book to read what God has had written in it, back and front. Only the Lamb of God, after having been slain and resurrected, can then appear to unseal it, and all heaven worships him equally, alongside the Almighty. Both of them share that one throne (Rev. 22:3), which has the seven-fold Spirit of God around its circumference, the same Spirit also being the seven eyes of the Lamb of God. This is what we learn from both chapters. And don't forget either, that the Lamb created everything, and everything was created for him, as shown elsewhere in the scriptures. That's not a contradiction, any more than this question suggests re. Rev. chs. 4 & 5. Trinitarians see how both are true. Non-trinitarians never will.

According to Trinitarian interpretations of Revelation 4 & 5, it is God and the Lamb being worshiped because the Father is God, the Son is God, and the Holy Spirit is God - one God. To worship one is to worship the others.