...IIRC the maximum short term (takeoff) thrust rating of each of twin engines has to exceed what's required per engine in 3/4 engine planes, so in normal operation there's more in reserve. (IOW they're running at lower % of maximum - less stressed) There's also an offsetting factor, all other things being equal you're twice as likely to have uncontained engine failure with 4 engines.....
That was kind of the problem first time round with the RR's - they are fine when under-stressed in cruise but they wouldn't last long enough on one engine at wide open throttle.
I'm not so sure I agree with where your unconstrained engine theory is heading. There is no question that 4 engines is better for your survival than 2. ETOPS 240 is a very good reating for a twin jet (ETOPS 180 is more common) and is hard work for an airline to maintain, but the standard B474-8 is rated at ETOPS 330 straight out of the factory, so the regulators are far more confident the 747's ability to get you there after an engine failure (or 2 😃) than a twin jet.
I don't know how a 747 could be ETOPs certified - it stands for Extended-range Twin-engine Operational Performance Standards, so there's no such thing for a 4 engine plane.
Yes all things being equal, then 4 engines should probably be safer than 2, but there's only a few 747s left, the A380s are aging and not going to be replaced, and I expect that modern twin engine planes record is probably *much* better with more modern engines than when the 747 ruled the skies. With an exception, Queen President Ivanka's new AF1 planes will have new generation engines, and under his eye, she'll be safe as.