4000000000 was a number.
The Twelfth Doctor gave his age as four billion when meeting an earlier incarnation of the Master. (COMIC: Doorway to Hell) This was because he had spent around 4.5 billion years trapped in his confession dial, according to the General, living out the same experiences and repeatedly dying to complete the cycle and eventually break free. (TV: Hell Bent)
The Fourth Doctor gave the population of Earth in 1977 as "four thousand million." (TV: Image of the Fendahl)
Isolus travelled in groups of about four billion. (TV: Fear Her)