Complex sentence: On April 6,2014, at WrestleMania XXX, he defeated The Undertaker and ended his undefeated WrestleMania streak, and on August 17,2014, at SummerSlam, defeated John Cena to become the WWE World Heavyweight Champion for the fourth time.
Split into simpler sentences: On April 6,2014, at WrestleMania XXX, he defeated The Undertaker and ended his undefeated WrestleMania streak. On August 17,2014, at SummerSlam, he defeated John Cena to become the WWE World Heavyweight Champion for the fourth time.