They have plans for a long-range passenger jet that will have these basic
engineering parameters and economic objectives: It will be a commercial
supersonic jet with twice the speed
of the Concorde [Mach 4 instead of Mach 2]; twice the range; and twice as many passengers onboard as the Concorde.

Yes, it would have to have twice the range and carry twice as many passengers to be economically viable, unlike the Concorde, but I find the engineering
parameters fascinating and exciting.

Twice the speed of Concorde?

At Mach-4 you'd be travelling faster than the SR-71 "Blackbird", and at such a speed you'd have to fly at an altitude
of anywhere from 20-25 miles up [The edge of space] where the Earth appears curved, the atmosphere appears as an arched horizon-band, and the sky above is black.

And just to show that the Japanese are serious, their engineers are apparently in the Australian desert right now ready to test launch a large-scale model to supersonic speeds by rocket.

Jacob