Here are some of my notes from Ibn Rajab Hanbali's risalah in refutation of those [non-mujtahids] who don't follow the four schools. "Preserving the Legal Judgments: Early on, there were many exercising independent juridical judgment (ijtihad) however this did not persist into the later generations. The people came to rely on scholars of vast understanding for rulings. Those that came after outlined their juridical methodology, provided details and connected these back to the principles of the school. If this had not been the case, then fools with high opinions of themselves might make mischief among the public. In a similar manner to making the community recite from one mode of recitation (during the Caliphate of Sayyidunā 'Uthmān (Radī Allāhu 'Anhu)) to avoid dispute and confusion in the public, the schools of jurisprudence were the established for the best interests of the public. The reason we don’t go outside the four schools is that someone could invent a position and attribute it to an earlier scholar. Unlike the system of checks we have in the four schools, ensuring an authentic transmission back to the scholar in question would be difficult – even if they twist the position to suit their whims. Moreover, even if a person is trying to follow a school outside of the four, we don’t know if the school they follow has been corrected or if a person misinterprets an Imām’s statement."