Issue with Rendering Large Texture

In my game, there is a space overworld where the user explores planets etc. The backdrop for the overworld consists of six 4096x4096 textures that have been tiled. The original textures were modified in photoshop to tile together seamlessly. In windows or photoshop, the textures look fine and show no distortion. In unity however, the textures have a visible blockiness to certain areas, with pixelation and distortion.

The effect can be seen in the screenshot here: http://imgur.com/eRrS2TK

I am wondering if this is an issue with the textures, unity, both, or some other external factor? I have tried nearly every combination of options in the texture menu without any progress. The camera is perspective and I'm using directional lights. Any advice on troubleshooting this would be appreciated.