4M what? Delhi doesn't have 4M cars. It's pretty much bordering on 9M cars, 10M if you include the greater NCR area.
The closest number I can find is 31.8L daily ridership in the metro which seems reasonable. The metro had an annual ridership bordering on 1B in 2017-2018.
Given a population of 20M, the whole city doesn't travel at once.
Add on other factors like -
* 30% people (kids, housewives, elders etc etc) do not have a significant commute.
* Then a significant portion of the population has a commute less than 5 kms for which taking metro makes less sense - this is evident in the gargantuan numbers of e-rickshaws, tuktuks, tempos seen on indian roads which primarily serve distances upto 5 kms and are nary unoccupied during peak times.
* Then another significant chunk has commutes smaller than 10kms. Only a small percent has any significant commute.
* Plus based on other factors I know, and which would be too tedious to link here, metro is the third most occupied transport behind buses (first) and small transport like e-rickshaws etc.
Delhi doesn't need to build more, in non-peak times you'll find plenty of space. Even in peak times, even though you'll be uncomfortable, you'll still reach your destination within 10-15 mins of what it would take in non-peak time. The current plan is well developed till 2021 and would probably serve the needs of the city with sufficient capacity, though not necessarily comfortable.