Have you done all the game updates, do you use Geforce Experience, and do you download new Game Ready Drivers when they release? My game was doing this as well the first week after launch. I can run 4K/60 at max with some headroom, but the game kept switching to 1440/60 dynamic/optimised. I've noticed people are having issues with the Nvidia overlay settings messing with the game as well. If you have that running, disable it for FH4. Go through the NVidia Game Ready Drivers in GeForce Experience and see what the settings are, if they match what the your game is being switched to, there's your issue. I'm not sure how GRDs choose their settings, but everything I have is water cooled/refridgerated, and pretty aggressively OC'ed, so I can run well above the factory specs NVidia has on my components. By disabling it for the game it won't over-ride your choices until the next driver update which will reset them again.
If you're running an AMD card, I have no idea what to look at.