The reason for the existence of the four Madhhabs
Shaykh ^Abdullah al Harariyy, May Allah endow great mercy on him, said:
The reason for the existence of the four Madhhabs-Schools of thought is that there are no statements in the Quran and Hadith that cover all the cases the human beings need.
During the life of the Prophet, sallallahu ^Alayhi wa sallam, when the Muslim encountered an issue that required a judgement he used to seek the Prophet for the ruling of his case. Alternatively he would go for that purpose to someone among the companions who accompanied the Prophet.
When the Prophet passed away the people differed in the way they understood some verses of Quran that bare two meanings; some companions expressed an opinion about these verses that they believed to be correct while others said otherwise that they believed to be correct; both applied what they saw correct without any of the two groups wronging the other.
People who learned knowledge from the companions, and came after them, also differed about certain cases and each said about these cases what conforms to his Ijtihad-mental endeavor without moving away from the overall sayings of the companions. Moreover, people, who learned knowledge from those, emerged who also had different opinions among themselves regarding cases and some said what they saw was correct while others said something different that they saw was correct. The same occurred among people who came after those people.
Similar to what happened in the past, differences in opinion occurred among the people of every century. Also differences about new cases did occur among them that had not occurred in the previous centuries such as in the following example:
<After the passing away of the Prophet, someone died leaving behind brothers and a grand father. Abu Baker’s Ijtihad was that the grand father inherits whereas the brothers do not inherit because the grand father has the same standing as the father; his analogy was that Just as the brothers do not inherit if the father was alive Abu Baker saw that the brothers do not inherit in the presence of the grand father.
However, Zayd Ibn Thabit and ^Aliyy Ibin Abi Taleb saw the case differently and said the brothers and the grand father share the inheritance.>
Those who came after the companions also differed about this case and the Ijtihad of some conformed to that of Abu Baker’s while the Ijtihad of Ashshafi^iyy conformed to that of Zayd and ^Aliyy.
There were Mujtahids other than the famous four namely Ashshfiyy, Malik, Abu Hanifah and Ahmad, however, their Madhhabs-Schools of thought were not recorded so they vanished after the death of their thinkers and those who applied them.
The famous four Madhhabs-Schools of thought Ashshfiyy, Malik, Abu Hanifah and Ahmad did not vanish and remained in circulation among their followers so the people ended up associating with one of the four Madhhabs.
Had the other Madhhabs been recorded it would have been permissible to follow them.
Nowadays there are no recorded Madhhabs, beside the four, of the companions or who came after them among their followers and the followers of the followers and others.