In order to discuss the four schools of thought in Islamic Law, we must first understand that there many great jurists, like al-Shâfi`î, Abû Hanifâh, Mâlik, Ahmad b. Hanbal, al-`Awzâ`î, layth b. Sa`d, al-Thawrî, and Ibn Jarîr al-Tabarî. All of these were independent jurists capable of juristic reasoning (ijtihâd).
However, the approaches of only four of these scholars became established. They are the approaches of al-Shâfi`î, Abû Hanifâh, Mâlik, and Ahmad b. Hanbal. The schools of thought based on their approaches to jurisprudence have lasted and have continued to be developed. Each school has had many adherents who have studied Islamic Law according to their chosen approach and have written major legal works based on it.
As for other schools of jurisprudence, they have not been developed nearly as much as those four schools nor have they become anywhere nearly as widespread.
However, this does not mean that those four schools of jurisprudence have a monopoly on the truth, though almost without exception the truth can be found within them. The reason for the continuation and proliferation of those four schools is that most of the great jurists since the third century of Islam have been affiliated with one or another of them.
The earliest scholars, the Salaf, who lived before the four schools of thought had been firmly established, were more like the founders of those schools. They used their own juristic abilities and derived the laws of Islam directly from the textual evidence. They would choose whichever opinion they saw had the strongest evidence to back it up. All of them sought to follow the truth. Sometimes they would be correct in their judgments and sometimes they would be mistaken.
After the four schools of jurisprudence became firmly established and settled, most jurists began to work within one or another of them. These scholars all had different abilities.
There is nothing wrong if a Muslim wishes to follow one of these established schools of jurisprudence as long as he does not become chauvinistic and biased to it. This is especially important when an opinion of his school is shown to go against clear textual evidence.
The words of the Prophet (peace be upon him) take precedence over the opinions of anyone else, no matter who they might be. The Prophet (peace be upon him) was protected from error in matters relating to Islamic Law. This cannot be said of anyone else.