QAN, I don't follow you. With 4 engines, there is slighly more than twice the probability of an engine failure than on a 2 engined aeroplane. As the 2 engine aeroplane operating under 180 mins EDTO rules has comparable remaining redundancy (electrical/hydraulic/pressurisation) on a single engine as does a 4 engine aeroplane operating with one inoperative.
If what you mean is that a 2nd engine failure on a 2 engine aeroplane will be catastrophic, then I see where you are going with your train of thought. Without getting too far off topic, new engines, such as the GE90, are extremely reliable. An old engine on a 4 engine aircraft that has been badly maintained will probably not develop full Max Continuous Thrust. Further, operation at MCT may overload the engine to the point where a second (or 3rd and 4th!) engine failure occurs. Assuming terrain clearance is ok, it may be prudent to operate the functioning engines at less than MCT during a diversion or continuation senario.
At high weights, a double engine failure on a 4 engine aeroplane will result in a book figure driftdown to a shockingly low altitude (below 10,000'). Add issues such as increased drag from an old airframe...well, it could get pretty ugly. Due to the reduced performance of 4 engine aircraft compared to twins( see CAO 20.1.7B), a 2 engine approach in a 4 engine aircraft is far more hazardous than a single engine appraoch in a twin. Remember that aircraft performance is based on a book figure derived from testing a brand new aeroplane (even more reason why a quality maintenance provider should always be used). For these reasons, a twin is at least as safe as a 4 engine aeroplane. Probably safer - especially if the Twin is new.