The Millennium is 1000 years just after the Second Coming of Christ. Those who have been saved will be in Heaven and will “judge men and angels”. The “wicked” will be in their graves and will not be resurrected until the 1000 years are over; at tht time, they and Satan will face the final judgment
The Four Horsemen of the Apocalypse use a number of symbols (riders, creatures, colors, objects) which together show a picture of what will happen when God and the Angels judge humanity.
The Sealed Scroll is part of the Drama of Revelation Chapters 4 and 5, where the Judgment begins in Heaven. The Sealed Book is the Book of Life, which Satan does not want to be opened, but Jesus appears as a Lamb Slain and His sacrifice qualifies Him to open the Book.