From the practically incoherent book The Finished Mystery, a commentary on the Book of Revelation and a compilation of Charles Russell's teachings after he died*:
[Revelation] 7:9. After this, I beheld, and lo, a great multitude.—When the
Apostle tells us in 2 John 8, “Look to yourselves that ye lose not
those things which we have wrought, but that ye receive a full
reward,” he is teaching that a Heavenly reward may be gained
that is not as full as if a course more pleasing to the Heavenly
Father is pursued.
Instead of teaching that the saved of our race will all be saved
to the same thing, the Scriptures show two degrees or kinds of
Heavenly salvation, and two degrees or kinds of earthly salvation.
In the second chapter of Genesis the stream which went
forth from the Garden of Eden was divided into four parts. This is
a Scriptural recognition of the fact that from Adam, the original
fountain of life, will flow four streams: The Little Flock, who are
to sit down with Christ in His Throne; the Great Company, who
are to stand before the Throne, having the palms of martyrdom
but without the crowns of glory; the Ancient Worthies, the Jewish
fathers, Abraham, Isaac, Jacob, Daniel, etc., who are to be
made princes in all the earth; and the world of mankind, who will
constitute the subjects of the Kingdom over which the Ancient
Worthies will rule. The same lesson is taught in the division
of the Levites into four camps, each located on a different side
of the Tabernacle. (Num. 3:15; F. 128, 129.) It is also taught
in the Apostle's statement in 2 Tim. 2:20, that in God's great
House there will ultimately be found four classes of vessels to
His praise.
*Note: If you find a PDF of The Finished Mystery use the scripture cited in Revelation as a guide instead of page numbers as the numbers differ in different versions of the book.