To meet the power needs of an aircraft with the 747's weight and carrying capacity, four engines were required. They were also required for long over ocean flights, to insure that the aircraft could make landfall in the event of a loss of an engine...

Technically, a Boeing 747 is designed with four engines to ensure maximum safety and performance at its maximum takeoff weight. However, it is capable of maintaining altitude and flying on just two engines once it is already in the air, provided the weight is managed. In an absolute emergency, it could potentially glide or stay airborne for a limited time on just one, though it wouldn't be able to climb or maintain long-term flight. The four-engine design was a product of the 1960s, a time when engine reliability was lower and "ETOPS" regulations required more than two engines for long over-ocean routes. Modern 747-8 models are incredibly robust, but the aviation world has shifted toward twin-engine jets like the 777 and 787 because they can now do with two engines what the 747 needed four to accomplish.