Big terrain problem. Chopping them into smaller doesn't look well!

I'm trying to make a big terrain (16Km x 16Km) with a texture of 2 meters per pixel. This forces me to make a 4 terrains of 4096x4096. A bigger texture size is not an option, so I have to chop it.

This is the result...

www.postimage.org/image.php?v=TsFfN1J

As you see is totally unacceptable. The joins are dark shaded!

How can I do this?