Texture painting

Hi all,
since September i’m trying to texture a blender model, without success.
it’s basically a big island, i can’t make it in unity (with unity terrain) because i need a precise shape, it’s about 2km^2.
i’ve tried using a single 4096*4096 texture, resolution is too low.
i’ve tried using 4 4096-4096 textures, resolution is still too low.
i haven’t tried using 16 textures, this would be too long to do, and probably, still looking bad.
i’ve tried searching and searching about vertex painting, i’ve tried vertexPainter by pixelstudio, but it won’t work (nothing happens when i press “paint”, i’ve sent an email to the developer, but with no answer).
now i’m trying to learn how to program shaders, but i don’t think that this is going to solve my problem.
can anyone please give me a hint???

you will need 5 textures of a size 256 by 256.

First one is an rgba texture, will serve as a filter.

Other four are rgb textures with the earth, rock, lava, grass whatever.
They have to be seamless textures, so when you tile them, there will be no seams.

After that, you will need a shader which will allow a certain amount of 1-4 textures show through depending on the strength of a corresponding channel of the filter texture.

Let’s say at a given texel of a filter texture the value is as follows (1.0,0.0,0.0,0.0). THis will imply that you want to have a texture1 reveal at the particular filter pixel. Other 3 textures do not contribute.

That way, you “show through” your four textures at particular spots, according to the filter texture.
After that you will need an editor script which will allow you to set the values of the filter texture in Editor, so that you don’t have to jump back and fourth into photoshop to adjust the filter texture.