Splitting Terrains and getting the textures to match up.

So I am working with a large map around 20kx20k. I painted my textures and got everything just about where I want it. Since the map is so large I decided to do the textures before I split them into many different terrains. so one split in I have four 10kx10k. When I split the initial terrain I got this problem where the textures no longer match up and I am not sure how to get them to match. Any advice on how to get them to match or ways to improve detail density for a large map would be helpful. If you have other quests and or suggestions to making this map feel big would be helpful. In the end I would like 16 5kx5k terrains with seamless textures.