sorry for the late reply, i did not get any notification... as far as your problem is concerned: i tried to reproduce it using unity 5.4.2. and the included demo terrain. turning on "precomputed realtime gi" and setting the terrain to "lightmap static" (directional mode) did not break lighting on detail texture 4 (which uses a far distance tiling value of 0.1). maybe i am missing something... which version of unity do you use? anything else i have to set? this is super strange and makes any try of reproduction even more difficult. so i more or less flooded the terrain with detail texture 4 – but the shading kept looking fine.