Toronto Blue Jays designated hitter Otto Vélez hits four home runs in a doubleheader against the Cleveland Indians at Exhibition Stadium. In the first game, he hits a grand slamin the 1st inning, followed by a two-run homer in the 8th, and a game-winning solo shot in the 10th. In the following contest, he clubs a three-run home run in the 1st inning. Overall, he has 10 RBI for the day.

On May 4, 1980 — Toronto Blue Jays designated hitter Otto Vélez hits four home runs in a doubleheader against the Cleveland Indians at Exhibition Stadium. In the first game, he hits a grand slamin the 1st inning, followed by a two-run homer in the 8th, and a game-winning solo shot in the 10th. In the following contest, he clubs a three-run home run in the 1st inning. Overall, he has 10 RBI for the day.

Source:
Baseball Reference May 4