The four Horsemen of the Apocalypse, as they are commonly called, are four men (whether physical, spiritual, or metaphorical) on four horses. These men are given different tasks to complete when the Lamb brakes a seal on a scroll that no one could break. The first four out of seven seals involve these four men. John the Apostle saw these men on their horses in his vision that is recorded in the Book of Revelation.
When the Lamb broke the first of the seven seals on the scroll, the first of the four Living Creatures said "Come". Then John saw a white horse with its rider holding a bow (arrows are not mentioned). The rider was given a crown and rode out like a conqueror determined on conquest.
Fiery Red Horse
When the Lamb broke the second of the seven seals on the scroll, the second Living Creature said "Come". Then John saw a fiery red horse with its rider being given power to remove peace from the earth, cause men to kill each other, and was given a large sword. Some people believe this is a reference to a world war, perhaps World War Three.
When the Lamb broke the third of the seven seals on the scroll, the third Living Creature said "Come". Then John saw a black horse with its rider holding a pair of scales. Then John heard a voice from among the Living Creatures said that about kilogram of wheat (which currently costs about 13 cents) would cost a day's wages (which is about $56) and about 3 kilograms of barley (which costs about 38 cents) would cost a day's wages. Then the voice said not to harm the oil nor the wine.
This horse and rider would cause major inflation on the earth. After a major war from the fiery red horse and its rider would cause this major inflation of prices due to war-stricken countries having difficulty to produce enough crops. The war would cause a high demand of scarce food.
When the Lamb broke the fourth of the seven seals on the scroll, the fourth Living Creature said "Come". Then John saw a pale horse whose rider was named Thanatos (Greek for "Death"). Following the horse and Thanatos was Hades. The power to kill ⅓ of the human population with sword, famine, plague, and the wild beasts of the earth was given to them.
These three would cause lots of death across the earth as an aftereffect of the war brought about by the fiery red horse. The war likely would cause a major increase in crime, especially murders. If the war is nuclear, then the nuclear radiation would be the cause of the plague and famine. The war may also drive wild beasts out of their natural habitats and cause them to lash out against people worse than happens today.