The new chapter of the Monster Hunter saga debuted only three days ago on the Nintendo Switch, and it was a great debut. Capcom indeed said of having sold over four million copies of the game, between digital and physical.
To give a term of comparison, in January 2018 Rise's predecessor, Monster Hunter World, had sold five million copies, but unlike Rise, it was available for both Xbox One and PlayStation 4.
However, there is a clarification on the figure. The four million copies were sold by Capcom, but it is not certain that they have already been retailed.
The statement released by the company speaks of copies "Shipped", not "sold", and includes physical copies sold to retail stores, regardless of whether they actually sold them themselves.
However, the difference should be minimal. In fact, there are no reports of shelves full of unsold copies. In reverse, in Asia the game is completely sold out.
Capcom would also already be at work for expansion that increases the number of available creatures, and should make it available towards the end of April 2021.