What does the fourth Seal and the pale horse represent?
The fourth seal shows a pale horse whose rider is named Death, with Hell following close behind, and together they are given power over a fourth part of the earth to kill by sword, famine, death, and wild beasts.
When the Lamb opens the fourth seal, John sees a pale horse whose rider is named Death, and Hell follows right behind him. The pale color of the horse points to sickness, decay, or the corpse like pallor of death itself. Power is given to this rider and to Hell that follows him over a fourth part of the earth, so that they can kill through the sword, through hunger, through death itself, and through the beasts of the earth. This shows that the fourth seal brings a wide reaching authority to take life by many different means at once.
This connects to what is revealed about Jesus in Revelation 1:18, where He says, "I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death." Jesus holds the keys that lock and unlock hell and death. The Pale Horse Rider is the one given the authority to release death and hell upon the earth, and this is seen worked out later through events such as the stinging locusts and the two hundred million army in Revelation 9.
It is also important to understand who this rider actually is. The four horses and their riders, including the pale (or "grisled") horse, are identified in Zechariah 6:6 as spirits that stand before the presence of God. They are angelic spirits, not devils, and not symbols for earthly nations. This means the judgments that come upon mankind through the fourth seal are ultimately determined and permitted by God the Father, since even the antichrist and his terrible rule over the earth must first be allowed by these four horses and their riders. As Daniel 4:17 and 4:35 make clear, God rules in the kingdoms of men and gives authority to whomever He chooses, and none can stay His hand or question what He does. So the fourth seal, with its pale horse, death, and hell, is a preview of a coming season of widespread mortality that unfolds under God's own sovereign decree, carried out by His angelic host rather than by the devil acting independently.
The scriptures
And when he had opened the fourth seal, I heard the voice of the fourth beast say, Come and see. And I looked, and behold a pale horse: and his name that sat on him was Death, and Hell followed with him. And power was given unto them over the fourth part of the earth, to kill with sword, and with hunger, and with death, and with the beasts of the earth.
I am he that liveth, and was dead; and, behold, I am alive for evermore, Amen; and have the keys of hell and of death.
The black horses which are therein go forth into the north country; and the white go forth after them; and the grisled go forth toward the south country.
This matter is by the decree of the watchers, and the demand by the word of the holy ones: to the intent that the living may know that the most High ruleth in the kingdom of men, and giveth it to whomsoever he will, and setteth up over it the basest of men.
And all the inhabitants of the earth are reputed as nothing: and he doeth according to his will in the army of heaven, and among the inhabitants of the earth: and none can stay his hand, or say unto him, What doest thou?
From the books
This answer is drawn from these passages in the Mary Banks Faith Library.