Earlier we learned a case where Rabbi Eliezer stated his opinion, but Rabbi Yehoshua disagreed with him. In regard to this, Shmuel said, “The law in general follows Rabbi Eliezer in four cases, and the above is one of them.” But in each of these cases, the ruling itself included the words, “And the law is like Rabbi Eliezer.” Why did Shmuel have to teach us the obvious? – That is exactly what Shmuel is telling us: just because the earlier ruling (Mishna) states that the law follows a certain Sage, we cannot say that this is necessarily true, since later rulings (Talmud) may change this, or establish it in only a specific limited situation. Thus, we cannot rely on our partial learning, but must have the totality of all the rules in front of us.
The Talmud then lists the other three cases, dealing with the discharge of a woman, discharge of a man, and the outside surface of a vessel that has become ritually impure through a liquid – that it can make other liquids impure, but not foods. But aren't there more cases!? – True, but Shmuel meant only in the laws of purity. But even there, there are more!? – Shmuel meant only cases where nobody else supports Rabbi Eliezer, and the law follows him based on his authority alone.
Art: Jan van de Venne - A Rabbi Reading