Actually, the customer required four because with the reliability level of 1960's engines, it was deemed unsafe to cross the pond on fewer. Bigger engines could have been built at that time (or shortly after) but no passenger wanted to trust fewer than four donks.
In fact there was something of a tempest over McD-D & Lockheed proposing trijets for overwater flights. Pan Am and BA were especially reluctant - the early sales of trijets were for transcontinental carriers. ETOPS? Don't make me laugh!
What has happened in the three-plus decades since then?
Performance trend monitoring and other on-wing diagnostics can detect an incipient failure well before the donk coughs. Engine time on wing has gone from a thousand hours to ten thousand or more. The statistics are in a new universe. A second unrelated engine failure in one flight happens at a rate measured in decades. In fact the "typical" flight crewperson will probably never experience a true engine failure in a career of professional flying.
To reiterate: an engine failure (sans fire warning) is not an emergency. When it happens, you survey the situation and make a decision. Then you follow through.