Bobby Lowe of the Boston Beaneaters becomes the first player to hit four home runs in one game, in a 20 – 11 win over the Reds in the second game of a doubleheader. Lowe hits two of his homers in the 3rd inning.

On May 30, 1894 — Bobby Lowe of the Boston Beaneaters becomes the first player to hit four home runs in one game, in a 20 – 11 win over the Reds in the second game of a doubleheader. Lowe hits two of his homers in the 3rd inning.

Source:
Baseball Reference May 30