There have never been ay indication that there will ever be a Witcher 4 game. In fact, we already know that we will never be playing as Gerald anymore but CD Projekt Red did say that the end of Gerald does not mean the end of the Witcher universe and the fans are hoping that there will be more in the future.
However, even if CDPR does decide to come out with a new The Witcher 4, we might have to wait until 2020 before we actually get to see the game. When talking about the upcoming Cyberpunk 2077 game, CDPR reveals that the game will only be ready between 2017 to 2021 which means even if they do decide to work on a new Witcher game, we will only be seeing it after 2020.
The wait might be long but with all the money they got from the Polish Government, the next open world game from CDPR should be amazing. CDPR got all four of their proposal approved by the government.