Is a Mach 4 Airliner Possible?
Above 40,000ft, or thereabouts, a human cannot survive even if breathing pure oxygen. The pressure they're breathing has to be higher (equivalent to a lower altitude) for enough oxygen to enter the bloodstream for survival.
This is why U2 / SR71 pilots wear / wore full pressure suits, in case of cabin depressurisation (and ejection). Pilots of jets like Typhoon can wear partial pressure suits, that can squeeze them around the chest to stop them exploding when breathing air at > ambient pressure in the event of depressurisation. On the odd occassion aircraft like the EE Lightning got up to U2 heights, they were effectively taking a chance that they'd not get a cabin depressurisation when up that high.
So hang on a mo. What did Concorde do about depressurisation? No special clothing was required, pilots and passengers just wore ordinary garb?
The answer is that, for Concorde, its cabin pressursation blowers and air con could keep the pressure inside high enough to be able to breath and survive, even if two whole windows had blown out at 60,000ft / Mach 2. Had that actually have happened, it would have been extremely noisy inside and very uncomfortable, but the occupants would survive long enough on oxygen masks for the aircraft to descend to a more accommodating altitude.
For a Mach 4 airliner, I can't see that flying below 90,000ft. To avoid a pressure suit it'd require a similar spec to Concorde's, but with at least another 30,000ft drop in pressure. That might not be so very different - there's barely any air at 60,000ft as it is. So perhaps that aspect of a Mach 4 airliner - ordinary cloting - is possible.