Revelation 6:8, "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."

As I understand it ...

1. Each of the 4 horsemen represents one of Satan's principal fallen angels that are given power over the earth, after the crucifixion of the Lamb (Jesus) who opens the seals.
2. Whilst each horseman is a spirit - there are people, philosophies, religions etc. through which each spirit manifests it's power.
3. Each horseman has power over 1/4 of humanity

The horsemen were released in chronological sequence, starting from the replacement of the Apostolic Church by the Catholic Church (~400AD).

4. White Horseman = Constantine, Catholicism, apostate Protestantism i.e. fallen Christianity - about 1/4 of humanity
5. Red Horseman = (? Atilla the Hun) Socialism, Atheism
6. Black Horseman = (? Odoacer) Capitalism, Worldly-ism
7. Pale/Green Horseman = Mohammed, Islam - about 1/4 of humanity

After the 4 horsemen, then there is a very long period of time (5th seal) - that's why the saints ask how much longer. During that period true Christianity will be attacked by the 4 different principal fallen spirits i.e. fallen Church, Atheism, Worldly-ism, Islam such that most of mankind will be under the control of these 4 spirits.

We are currently in the time of the 5th seal. The 6th seal will be very easy to spot - the sun will be darkened, but not by the usual lunar eclipse - yet to happen, but when it does, it will be undeniable.

in Jesus' Name,
Lester