Sounds like you might have a couple of different problems you’re going to have to deal with.
A big one is that all the height maps are going to have to be normalized to the same scale across all of them (i.e. whatever the highest point is across all of the height maps needs to have a white value of 1, and whatever the lowest point is across all of them should have a value of 0). If each heightmap is normalized to its own scale, you’re never going to get them to line up right in Unreal.
If you can manage to do that, the best approach from there is to merge all the heightmaps into one image file in an image editor and then import the landscape into Unreal. But if the resulting heightmap is super high res, you’ll probably run into the max landscape resolution limit and won’t be able to import it, and you’ll be forced to cut the heightmap up again and import separate landscape actors and align them manually (this may not be necessary any more in 5.4, but I haven’t tested it. I do have a project where I had six separate landscape actors created from 1m resolution USGS heightmaps and was able to line them all up, so I know it’s possible to do that way, but it was a real pain)