Problem tiled landscape

i have created in WM a map 32km x 32km, with a resolution of 8129 x 8129 and 4x4 side, but i don’t have enough power to do tiled build (i7 6700 / 16gb ram) and then i minimazed it in a 16km x 16km, with a resolution of 4033x4033 and 4x4 side, but, again, i don’t have enough power and then i minimazed again in 8km x 8km, with a resolution of 2017x2017 and 4x4 side. Now, for create the first map 32km x 32km i need create 16 tiled landscape 8km x 8km.
Now i have create 4 tiled landscape 8km x 8km,

now the question, how can i “attach” the 4 tiled landscape 8km x 8km and make a map 16km x 16 km, i’ve tried to load all levels that i created and imported but i have problem with overlap map, it load all level one over the other, essentially i would like to return to landscape 32km x 32km with 16 tiled landscape 8km x 8km.
how can i do this?
thanks.

The generated files are just images, you can patch them together, or split them apart in a paint program such as Photoshop.