Siddhartha saw an old man struggling to walk, which made him realize the inevitability of aging and the sufferings that come with it. This sight made him contemplate the fragility of human life and the impact of time on the body.
As Siddhartha continued his journey, he saw a sick man. This sight showed him the suffering that comes with illness and the helplessness of those who are afflicted.
Siddhartha witnessed a dead man being taken to the cremation grounds, which made him contemplate the inevitability of death and the impermanence of all things.
Lastly, Siddhartha saw a holy man, who was a wandering ascetic, and this sight made him think about his own search for the meaning of life. He realized that the happiness and fulfillment he was seeking could not be found in the pleasures of the world, but in spiritual pursuits.
After seeing these four sights, Siddhartha became realised he had been living in extreme luxury and was ignorant to how others were living. He decided to seek the truth and learn how to overcome suffering.