On April 10, 2004, Rentería had four hits in a 10–2 victory over Arizona. On June 9, against Mark Prior, he hit his third career grand slam in a 12–4 victory over the Cubs. He had a ten-game hitting streak, his best of the year, from June 22 to July 3. During the streak, on June 26 against the Kansas City Royals, his RBI single in the tenth inning gave the Cardinals a 3–1 victory. Also, on July 2, he scored four runs in an 11–2 victory over the Seattle Mariners. He was one of three Cardinals selected to the All-Star Game. He had four hits again on July 15 in a 7–2 win over Cincinnati. On August 8, he had five hits in a 6–2 win over the Mets. He had five RBI on August 22 in an 11–4 victory over Pittsburgh. Rentería finished the year with a .287 batting average, and he had ten home runs, 72 RBI, and 84 runs. The Cardinals made the World Series that year, but they were swept by Boston. Rentería was the final batter of a World Series for the second time in his career, when he grounded out against Keith Foulke to end Game 4 as the Red Sox won their first World Series in 86 years. However, he was one of only three Cardinals position players to bat above .250 in the Series as he batted .333. Following the year, he became a free agent.