In this tweet posted by the official Ubisoft UK account, it has been confirmed that there is a 5th Farcry on the way.
The tweet links off to the official forum for the upcoming game. There have been no further details yet apart from Ubisoft asking fans if they’re ready for more adventures.
I am. As long as there’s no bloody magic floating about.
— Ubisoft UK (@UbisoftUK) May 16, 2017
Also worth noting is the Crew II has been announced in this Ubisoft forum post