The Lords fourth incarnation was Nara-simham This incarnation was demi-human- Half human and Half animal (lion). Nara means human simham means lion.

After his brothers death Hiranyaksibu became afraid for his life. He wanted long life and power. One day he got the idea to go to the forest and do meditation to get more life and also ward off any trouble with any animal or the Gods.
So Hiranyaksha went to the forest to do meditation. After a severe and devoted meditation for along time LORD Brahma appeared before him. He asked Brahma for immortality .But Brahma said no. He said all men when born have to die some time or the other and cannot live for ever. Brahma said "ask some thing else" Then Hiranyakasibu asked that he should not die either at the hands of any human or killed by any animal - in the day or at night. Brahma gave him the boon asked for and disappeared.

Even though his wife was pregnant Hiranyakasibu went to the forest to do penance. In the meantime Indra thought "If Hiranyakasibu gets the boon that he wants then we will all be in deep trouble. Hence I should kidnap his wife and keep her in jail". So when he returned after kidnaping Hiranyakasibu's wife Naradhathe sage came and stoped him. Naradha told Indra "you should not do this kind of coward acts. Hiranyakasibu is definitely going to get the boon and nobody can stop him. leave the lady with me and go to your 'lokha'. Indra left that lady with Sage Naradha and went off. Naradha took that lady to his abode and told her good moral stories. She would listen to him but sleep off. At that time the Sage would start to tell the stories of God Vishnu and also Prayers to God Narayana. The child in the womb of the lady listened to all the stories about Narayana and also the prayers. These got embedded firmly in the mind of the child-to-be born. The child when born was named Prahalada and he grew up as anardent devotee of Lord narayana.

But Hiranyakasibu had ordered all his subjects and all around him to pray to himself and not to any god. Hence was much annoyed by the devotion of his son to lord Narayana whom to God Narayana whom he considered as his enemy. Prahalada was ill-treated and tortured to forget god Narayana but could not get him to forget the Lord. Rakshasa Hiranyakasibu lost all his patience and one day in irritation asked prahalatha "Ok my boy where is that god Narayana ? Can you show me"? "yes"said Prahalada. He is everywhere and anywhere. The Rakshasa asked his son " pointing to a pillar "is he in this pillar ?" "yes he is in this pillar" said the son. Immediately the rakshasa took his huge club and hit the pillar. The pillar broke in to two pieces and appeared from it Narasimha neither a human nor an animal ie (half man and half lion) and took the Rakshasa on his lap and killed him with his claws. Thus came to end the evil called Hiranyakasibu. God took Narasimha avathar only to kill evil.
This was Lord's fourth incarnation to shave people they should be devoted to the God.