Where did you hear that?
It’s an assumption: http://bgr.com/2016/09/23/disney-star-wars-movie-release-schedule/
It’s not clear at this point whether a fourth Star Wars trilogy is in the works […] or whether Disney will focus on other characters from the saga.
So you’re right we don’t know yet if a fourth trilogy is planned, but it’s still a very high probability they’ll make it happen.