Is this because of something analogous to an expected value calculation? Like the increased probability of failure of a single engine when carrying 4, combined with the lower excess power per engine, means that the safety margin of a 4 engine plane is worse? Or is it something to do with the doubling of the number of critical parts that could fail? I'm struggling to come up with a workable argument why 2 is safer, it seems counterintuitive. I don't have enough of the puzzle.

EDIT: Never mind, I see you wrote more down below about this. It's increased risk of a catastrophic engine failure bringing down the entire plane, not an engine simply dying and forcing a landing. Four engines = twice as many chances for that to happen.