I faintly remember reading somewhere that the landscape system calculates a normal map from the height map to be used on the terrain (is that correct?). However, I have normal maps (from World Machine) that I would like to use to increase the appararent resolution of the terrain. Can I do this? Would I simply assign them to the normal slot of the material (using a terrain coords node for correct UV mapping)? Would I have to merge those terrain normals with the normal maps that come with the detail textures?
Yes you can. See the tutorial below for how.
Copy/pasted from another World Machine question I answered 5 mins ago.
This is a step by step tutorial. Worked perfectly for me.
Step by step for everything except the splat map creation, for which world machine has an example.
Sorry for bumping this, but I’m really interested in an answer 
If I wire my normal map texture (generated in World Machine) to the normal port of my Landscape material, the shadows get way too pronounced. I guess UE4 creates a normal map out of the heightmap (by finite differencing or so) and uses that, anything wired to the normal output is added to the internal normal map? Is there a way to bypass that? Also, the docs state that
" In addition, the full-resolution normal map of the Landscape is available. "
How? Thanks!
Yes you can. Just plug a landscape coordinate node to the normal map and set the UV scale to the size of your landscape.