But is it less safe than flying on two? Considering that it can actually fly the 60 minute flight path to a divert airfield on just one, flying on three would still leave you two more improbable failures away from declaring an emergency, while flying on two (A310, 777) puts you just one failure away from an emergency.
My original point was that it was a bad judgment for British Air to fly on three engines from LA to London. I once flew British Air. I will never again.
To answer your question about four engine craft versus two, you need to know what the reliability the engines are rated for each design. I would imagine that the Federal government requires a performance standard for airliner reliability. This means the designer of two engine aircraft have got to provide the same reliability that a four engine aircraft would have. They both have to have the same chance of meeting that standard. More reliable engines is one way to solve the design problem.