Frostbite has the best terrain system in the industry at the moment. There are several really helpful PDFs on that Epic can also take a look at. (If interested).
Battlefield 3 had 8k terrains while the game as a whole runs perfect even on some low end PCs. With Star Wars they even have massively used tessellation and layer blending on the terrains and still the game as a whole runs at a butter smooth 60FPS.
Epic did do some massive open world showcase with Kite Demo but never really took the tech to a usable state. There were many complains about how bad it ran but the blame was usually diverted towards the huge texture sizes and such things while in fact, unoptimized and old systems played the major role there. This can’t be a fix to come with 4.15 or 4.16. It will take it’s time and needs a lot of effort which I hope to see it happening sooner than later.