Hey kurylo3d,
It looks like the issue you are experiencing is a common limitation of the current shader model per landscape component. We introduced the Shared:Wrap texture sampler in 4.6 which allows for more layers and textures for your landscape material.
When you are reducing a weight-blended layer by holding down Ctrl + Shift while painting, it is not clear what layer should be increased to replace it. The current behavior is to uniformly increase the weights of any other layers. Because of this behavior, it is not possible to paint all layers completely away. Instead of painting layers away, it is recommended you choose the layer you want to paint in its place, and paint that additively.
Thank you,