I have the standard version of World Machine (the first paid tier). Whenever I make a 2k (2049) map, export the heightmap, and import it into Unreal, it looks great, but it’s way too small. If I go up to 4k in WM and export/import, Unreal’s Landscape import feature never goes above 2049 on the resolution (and the dropdown is gone – bug?). Is UE able to import larger than 2049? One workaround I have found is scaling everything by about 300%. This kind of works, but the nice landscape features go kind of flat.
That does not apply to landscape heightmaps although in general your advice is good. Four 4k landscapes are probably more manageable than a single 8k but that can depend on your project setup. For just visual testing and iteration there is nothing wrong with 8k but once you get into performance concerns and multiple people trying to work in the level, splitting it up will help.