You are going to receive a brief summary based on these two chapters. Please refer to the enotes links below to gain more information about them and the novel as a whole.
Chapter Three introduces Agbala, the Oracle of the Hills and Caves, who is consulted by people during times of hardship. We are told that Unoka, Okonkwo's father went to Chika, Agbala's priestess, once, and was told that the success of his harvest was dependent upon hard work. Unoka's failure to work like a man is because of his weak chi, or personal god, and thus he is ill-fated. Okonkwo therefore does not inherit anything from his father. Instead, Okonkwo makes his fortune by working for Nwakibie, a very important man in the tribe. Okonkwo asks Nwakibie for yams to plant and impresses him with his dedication and hard work. Nwakibie agrees and Okonkwo is given yams to plant.
Unfortunately, Okonkwo's attempts to show his diligence seem doomed, as the first 400 yam seeds he plants are fried by a severe drought. The second crop receives too much rain. This tragic year is something that becomes marked indelibly in Okonkwo. Unoka, as he grows older, likes to talk more and more, and tells his son that a proud heart can survive a general failure. Okonkwo becomes increasingly irritated by his father's words.
In Chapter Four, we are told that Okonkwo gains success and status through his hard work. However, he is rude to those who are not successful. Ikemefuna gradually becomes an inseparable part of Okonkwo's family, in particular being a good friend to Nwoye, Okonkwo's son. During the Week of Peace before planting crops, Okonkwo breaks a taboo by beating one of his wives because she neglected her duties. The priest of Ani, Ezeani, tells Okonkwo that this is a major sin as it could ruin the harvest for the entire clan. Okonkwo as punishment must bring one female goat, on hen, a length of cloth and 100 cowries to Ani. Okonkwo does this, but will not admit his crime in public. This makes other clan members say that he has no respect for the clan and for their religion. The "crime" of Okonkwo is discussed by all the people during the Week of Peace.
After the Week of Peace is over, the people work hard to prepare new farms and then cultivate yams, which are symbols of manliness and viewed as the kign of the crops. The process of yam farming is described. When Nwoye and Ikemefuna try to prepare seed-yams, they are punished by Okonkwo, who wants his son to be a great farmer.