Sony recently launched "My PS4 Life" allowing players to share some stats of their gaming experience.
Among other numbers, they also disclose the exact number of players who received some trophy. Combined with the % of people who obtained the trophy (this data is available for every game on the PlayStation website), we can estimate the total number of players in this game.
Here, 1,142,392 players earned the "Cradle" trophy in No Man's Sky. And on playstation.com they also reveal that 28,9% of all players got this trophy. Simple math: the total number of people who have played No Man's Sky on PS4 = 1,142,392 * 100% / 28,9% = 3,952,913.
The same trick can be done with every PlayStation 4 game - you just need to find a video where it's someone's rarest trophy. But better not too rare - 0.1% share doesn't give a good estimate.
Note: the number of players who have earned the trophy is not updated in real time, Sony probably took it somewhere in mid November. So it won't say much about recently released games.
P.S. In August 2018 ago number of players in God of War was calculated in exactly the same way when developers revealed the number of players who got the Platinum trophy.