Why Did Boeing Build The 747 With 4 Engines?

The Boeing 747 Jumbo stands out for being the most popular quad-engined commercial passenger jet to exist. It also boasted the longest production run of any commercial jet in its class. But why was it designed with four engines and not two, three, or even six? When answering this question, it is necessary to look back in time to the 1960s when the original 747-100 was developed.