Musar על חגיגה 28:4

The Talmud reports in Chagigah 14b that four outstanding scholars entered the "orchard," i.e. they investigated secrets, mystical dimensions of Torah study not given to everyone to understand. Only one of these four sages, Rabbi Akiva, returned from this investigation without any harm. Four people preceded these four Talmudic scholars in their desire to probe these mysteries. They were: Adam, Noach, Abraham and the sons of Aaron (who are considered as one, as a team). They all entered this "orchard" or vineyard.