I think it means it can do it with 1, but the fact it has 4 gives it great redundancy.

I'm not sure. In the case of 4 engines, it may be 2 is how they certify it. Specifically I think the case where both engines on the same wing fail (as the worst case other that losing 3)

The rating is taking into consideration that it has 4 engines. The term is just confusing because it used to only apply to twin engine aircraft.