Revelation 6:7 And when he had opened the fourth seal, I heard the voice of the fourth beast say, Come and see.
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.

The fourth of horsemen is a pale horse, which means he brought death and hell for those that this death killed were not saved. By it saying a fourth part of the earth it means over a billion people would die from the not only the war that was fought but also from the famine.