The Fourth Imam, Ali Ibn Al Hussain Zainul Abedeen (as)
Zain al abideen was the son of Imam Hussain (a.s). when he was two years old his grandfather Imam Ali (a.s) was killed, and so his uncle Imam Hassan became the Imam, when Imam Hassan (a.s) was poisoned his father Hussain (a.s) became the Imam.
Zain al abideen (a.s) was sick during the battle of Karbala and it was this sickness that saved his life because all the men on that day were killed. Zain al abideen became the Imam on the 10th of Muharram. His father Imam Hussain (a.s) was fighting alone on the battle field of Karbala when he with drew from the fight and went to the tent where his sick son was asleep. Imam Hussain (a.s) woke up Zain al abideen (a.s) told him that the story of Karbala was almost over, that he was going to go back and be killed for the sake of Islam. In those last tender minutes they had together Imam Hussain (a.s) transferred the Imamate (spiritual guidance of humankind) to Zain al abideen (a.s).
When the battle was over, Imam Zain al Abideen (a.s) along with the women was taken to Damascus, in Syria to face Yazid. During that time the remaining family of the prophet were treated badly and the women had their head covering taken off.
This small group of women and the sick Imam were taken to a large mosque in which Yazid sat on thrown with hundreds of people watching.
Yazid took this opportunity to try and humiliate the Imam and he said loudly to him that his father wasted his life by getting killed.
Yazid was the person who had ordered his father to be killed and who enslaved him, yet Imam Zain al Abideen (a.s) showed no fear. Yazid tried very hard to humiliate the Imam, but the Imam (a.s) did not care about what Yazid or his friends thought.
Bravely, the Imam (a.s) replied "My fathers death saved Islam"! Everyone was confused by this reply, just then the sound of the Azan could be heard saying "I bear witness that Muhammad (s.a.w) is the messenger of Allah (s.w) " The Imam then shouted to Yazid, "this is the way Islam was saved"!
This made everyone understand that if Imam Hussain (a.s) did not die in such a horrific way in Karbala at the hands of Yazid, then no one would have ever questioned Yazid and his way of life and the real Islam would have been forgotten. This is because Yazid used to do many haram (forbidden things) like drink alcohol, gamble and kill innocent people and he would not do the things Muslims have to do such as pray, fast and be honest.
Yazid then sent the Imam and the women of the prophet to live in jail. However, slowly people started to know that Yazid killed the Prophets (s.a.w) grandson and put his family in jail and they became very angry. This worried Yazid so he decided to set the prophets family free.
After he was freed Imam Zain al Abideen (a.s) went back to Medina to live there. In Medina, the Imam's(a.s) situation did not became easier, his heart stayed heavy with pain at what happened in Karbala and to make things worse Yazid's army invaded Medina soon after.
Yazid hated Islam so much that he made sure that anyone who tried to teach the true Islam or say anything about the Prophet and his family would be killed. This was a big problem for the Imam because his job was to guide and teach the Muslims.
The only thing he could do without getting killed was to pray towards Allah. So at night he used to go to the Mosque of the prophet and make beautiful whispered prayers to Allah. These prayers were recorded by his sons and spread to the Muslims in a book called "Al-Sahifa al Sajjadia". These whispered prayers were the Imam's (a.s) fabulous solution to the problem he faced, because although he could not give lectures openly to people, through his prayers he taught them the beauty of a true relationship the between man and his creator.
The Imam (a.s) also taught the human race their rights and duties towards each other and the world around them in a book called "Risalat al huquq". Both these works are now famous and still help Muslims get closer to Allah (s.w) and understand their place in the world.