Revelation­ 4:1 "After this I looked, and, behold, a door was opened in heaven: and the first voice which I heard was as it were of a trumpet talking with me; which said, Come up hither, and I will shew thee things which must be hereafter."

So many questions just getting past this first verse.

1. What open door?
2. Hereafter what?
3. Where in the timeline are we?
4. What event is taking place?
5. What is the official SDA belief on this?

We will look at each one of these in different lessons but lets first look at what the SDA church teaches.
I don't know if there is an official SDA belief on Revelation 4 and 5 or not but I've heard this just being a Praise Program In Heaven, The Inauguration Of Jesus, and of course the Investigative Judgment.
First off this scene is more important than to be just some sort of praise program.
Many don't like to entertain this being the Investigative Judgment and I'm not sure why.
So as we see below Stephen Bohr says Revelation 4 is the inauguration of Jesus.
You can watch just the first few seconds to see his belief or watch the whole thing if you want.

Here is a quote to understand the time of the inauguration of Jesus.

When Christ passed within the heavenly gates, He was enthroned amidst the adoration of the angels. The Holy Spirit descended on the disciples, and Christ was indeed glorified. The Pentecostal outpouring was Heaven's communication that the Redeemer's inauguration was accomplished. The Holy Spirit was sent as a token that He had, as Priest and King, received all authority in heaven and on earth and was the Anointed One. {TT 22.5}

Where do I stand?
I stand with those that believe this is about the Investigative Judgment.
Here are a few reasons for that.

1. We see an open door. Rev 3:8 & Rev 4:1
2. We see that the thrones are set. Dan 7:9-10 & Rev 4:2
3. Books are being opened. Dan 7:10 & Rev 5:9
4. Same number of angels. Dan 7:10 & Rev 5:11

I don't know how anyone can say Dan 7:9-10 and Rev 4 & 5 are two different events when the language is so similar.
Stephen Bohr tries to separate Revelation four from five which I think goes against the text.
This is the same event, first we see God and his throne set and then Jesus comes in and the books are opened.

I feel like we need to go back and look at the Investigative Judgment.

If you read Hebrews you will understand that there is a Sanctuary in Heaven. Heb 9:24
If you read Daniel 8:14 in connection with Lev 16:30 & Lev 16:33-34 you will understand that the Sanctuary needs to be cleansed which deals with the removal of sin.
That is the scene we see in Daniel 7:9-10 and which I also believe to be Revelation 4 & 5.
Lets look at a few more quotes about the Investigative Judgment.

At the time appointed for the Judgment-the close of the 2300 days, in 1844-began the work of investigation and blotting out of sins. All who have ever taken upon themselves the name of Christ must pass its searching scrutiny. Both the living and the dead are to be judged "out of those things which were written in the books, according to their works." {GC88 486.1}

Attended by heavenly angels, our great High Priest enters the holy of holies, and there appears in the presence of God, to engage in the last acts of his ministration in behalf of man,-to perform the work of investigative Judgment, and to make an atonement for all who are shown to be entitled to its benefits. {GC88 479.3} In the typical service, only those who had come before God with confession and repentance, and whose sins, through the blood of the sin-offering, were transferred to the sanctuary, had a part in the service of the day of atonement. So in the great day of final atonement and investigative Judgment, the only cases considered are those of the professed people of God. The judgment of the wicked is a distinct and separate work, and takes place at a later period. "Judgment must begin at the house of God; and if it first begin at us, what shall the end be of them that obey not the gospel?"

As the books of record are opened in the Judgment, the lives of all who have believed on Jesus come in review before God. Beginning with those who first lived upon the earth, our Advocate presents the cases of each successive generation, and closes with the living. Every name is mentioned, every case closely investigated. Names are accepted, names rejected. When any have sins remaining upon the books of record, unrepented of and unforgiven, their names will be blotted out of the book of life, and the record of their good deeds will be erased from the book of God's remembrance. The Lord declared to Moses, "Whosoever hath sinned against me, him will I blot out of my book." [Exodus 32:33.] And says the prophet Ezekiel, "When the righteous turneth away from his righteousness, and committeth iniquity, ... all his righteousness that he hath done shall not be mentioned." [Ezekiel 18:24.] {GC88 483.1}