Скачать или смотреть Unity Terrain Tile problem

I am wondering what the rules are for avoiding gaps in seams between terrain tiles.

I have a simple game at the moment which is 12 x 4 tiles of 10mx10m each.
I can click and adjust the height of the surrounding area I click on at runtime (this is quick because we are using such small Terrain sizes per tile.

However things start getting very complicated to adjust the heightmaps in the terrainData if somebody clicks between tiles, and even worse where the 4 corner points of 4 tiles meet. The situation I am finding so cumbersome in my algorithm that I end up with gaps between the 4 corners.

Is there a known way to handle this? Perhaps somebody has already solved this problem before? If not, what are the rules my height adjustment algorithm should follow for the points next to each other, but on different terrain tiles.

It's worth noting, that I have figured out how to handle the border between 2 tiles.... but I am finding it hard and just complicated to handle borders between 4 tiles.