Complex sentence: "The Fabulous Four" created a wave of popularity in the lower weight classes that kept boxing relevant in the post-Muhammad Ali era, during which Leonard defeated future fellow International Boxing Hall of Fame inductees Hearns, Durán, Hagler, and Wilfred Benítez.
Split into simpler sentences: The Fabulous Four created a wave of popularity in the lower weight classes that kept boxing relevant in the post-Muhammad Ali era. He defeated future fellow International Boxing Hall of Fame inductees Hearns, Durán, Hagler, and Wilfred Benítez.