My Question is on what logic is the shariah of all four Imams based? When did it happen i.e. what are the time gaps, from one to another and when did the first start before Khilafa or after Khilafa.
There are many issue is in and in it as in the deen in which there is a necessity to do ijtihad, and for this reason from time to time ijtihad is and has been done. In fact, in the time of the Sahaaba, this necessity arose and ijtihad was done based upon the principles of the Quran and the Hadith, and the formation of rulings based upon the evidences from these two sources. The differences in the outcome of rulings results from the difference in opinion regarding rules and principles of ijtihad and its application to specific cases. For this reason, all of these jurists are correct because they base their rulings upon valid principles and Allah has given them the ability to do so.
There have been many mujtahids, but only the jurisprudence of four imams have been properly saved and collected, and only these four can thus be followed and any rulings will be confined to the rulings of these four. After this, there can be no fifth position. Jurisprudence has thus become confined to these four and it is necessary to follow one of them. When two schools of jurisprudence exist in one time or place and there is not a big difference between the two and it will not make a difference between following one or the other than it is the decision of the individual to follow whichever imam he has most trust in. And because there are only four imams left, it becomes necessary to follow one of the four and it is also necessary to keep in mind that these four imams are from the time of the tabieen and the tabi tabieen.
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