One's a billionaire funding the mission; another a health care provider. The third will be selected at random through a sweepstakes, and the last seat will go to the winner of a competition.
In the new Space Age, you can buy a ticket to orbit - no need to have been a fighter pilot in the military or to compete against thousands of other overachievers for a coveted spot in NASA's astronaut corps.
In fact, for this mission, the first comprised entirely of private citizens, NASA is little more than a bystander. It does not own or operate the rocket that will blast the astronauts into space or the capsule they will live in for the few days they are scheduled to circle the Earth every 90 minutes. NASA has no say in selecting the astronauts, and it will not train or outfit them - that will all be done by Elon Musk's SpaceX.