The transition from 4 engines to 2 was accompanied by a great increase in systems reliability. Look up ETOPS for details. The trade off between safety (loss of airframe) and reliability( plane has all engines functioning ) is complex. A 2 engine aircraft is more reliable than a 4 engine aircraft, but a 4 engine aircraft can still fly safely with less than 4 engines. ETOPS made the design margins increase such that 2 engines would have identical safety and reliability.