Terrain advice

Hi
So i have settled on a world size of 20000x20000 or 20km. I have at this point set a low height resolution of 129 . But i will need parts with more resolution at least 513 maybe more. I am debating whether to split the terrain into 1km tiles which will be 400 tiles or to have multiple terrain with the 20km base underneath and a number of higher resolution tiles spread around…

I would prefer the 400 tile route but is there a way to stitch the terrain together also how do i place them so no gaps it will be very tedious is there some way to snap them together?

A reason why i went low resolution on the main map is i expect to use more than 4 textures. I read this will require the terrain to be drawn twice but when i tested with 12 textures the tri count, setpasses every stat in the stat window remained exactly the same as when i used just 1 texture. Haven’t used the editor much but i did not see any changes in it either. So why is it considered so bad to have more than 4 and when can i see the impact ?
Also i noticed that 600 is no longer the max height just set a height of 10000 is that a change in unity 5 or is my terrain just bugging out?
Edit. Actually changed my mind think the 20km low res terrain with smaller high res terrains is a easier fir. What is the downside? I s it a memory hog not sure where terrain impact the most.
Thx

Terrain Composer can help with such tasks (stitching, tiling, batch changing resolutions and sizes, etc.). Available at Asset Store. Very usefull.