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Remember, just because an engine can remain running at a particular speed does not guarantee that an aircraft it's in can go that fast.   Consider where the shock wave coming off the nose of an SR would be relative to its wingtips at a TAS of M4.25, let alone hypersonic, along with the fuel requirements.  Also,  a ground speed approaching M4 does not mean that the aircraft itself  is capable of  M4.  We are talking late '50s-early '60s technology.