8.3 billion is the most commonly cited figure for the number of people alive today. The riddle proposes that that number miscounts by exactly 4. Thus there must be a list of four people who are alive in some unique sense where they would not ever be counted. The clue that the additional 4 would be considered by "some" and not "an easy claim" means it is something believed but not necessarily verifiable.
Christianity, proposes four individuals who were taken to heaven with their bodies.
Any other such set of four doesn't really match "four people who are alive but not counted in the population of earth." Astronauts for instance. There are more than four and they are generally counted in the cited figure.
