Well the screenshots you've posted look rather nice, likely due to being resized so its difficult to see the problem.
A few points
You're textures are already lossy compressed via jpg so that might be having an effect.
Not sure why you disabled mipmaps, that means you are going to be using point filtering which can sometimes not look good, especially if you have large textures that are then downsampled in the render view. I'd recommend switching mipmaps back on, using trilinear filtering and increasing the ansio setting. Its also less efficient due to cache misses, though its difficult to know just how much of an effect it will have on performance as its dependant upon the project.
I'm not convinced that you need 4096x4096, not unless you are using something higher than 2560x1600 display, since the textures will have to be downsampled. Taken into account the use of point filtering (since you disabled mipmaps) I could imagine this might have a determental effect.
The only reason for using such large textures (IMHO) is if you are displaying on a very high resolution monitor (or spanning several monitors) or if you can zoom right ito the controls.
Unity settings to check
a. The texture size default is 1024, make sure you've increased the value to 4096 and hit apply.
b. Check the ansiotropic setting, though off-hand I can't remember if this requires mipmaps to have an effect.
c. Check the texture format, its likely to be using compressed (or DXT1/DXT5) and instead try truecolour or RGB24 or RGBA 32.
If you are still having problems, you may need to post a demo or link to some full size screenshots (don't embedded actual images in the question since they wil be so big) and perhaps a sample of one of the actual textures. That might make it easier to see the problem.
