Terrain Shader

Shader "Custom/4-Part Texture" {
    Properties {
        _MainTex0 ("Base (RGB)", 2D) = "white" {}
        //Added three more textures slots, one for each image
        _MainTex1 ("Base (RGB)", 2D) = "white" {}
        _MainTex2 ("Base (RGB)", 2D) = "white" {}
        _MainTex3 ("Base (RGB)", 2D) = "white" {}
    }
    SubShader {
        Tags { "RenderType"="Opaque" }
        LOD 200

        CGPROGRAM
        #pragma surface surf Lambert

        sampler2D _MainTex0;
        //Added three more 2D samplers, one for each additional texture
        sampler2D _MainTex1;
        sampler2D _MainTex2;
        sampler2D _MainTex3;

        struct Input {
            float2 uv_MainTex0;
        };

        //this variable stores the current texture coordinates multiplied by 2
        float2 dbl_uv_MainTex0;

        void surf (Input IN, inout SurfaceOutput o) {

            //multiply the current vertex texture coordinate by two
            dbl_uv_MainTex0 = IN.uv_MainTex0*2;

            //add an offset to the texture coordinates for each of the input textures
            half4 c0 = tex2D (_MainTex0, dbl_uv_MainTex0 - float2(0.0, 1.0));
            half4 c1 = tex2D (_MainTex1, dbl_uv_MainTex0 - float2(1.0, 1.0));
            half4 c2 = tex2D (_MainTex2, dbl_uv_MainTex0);
            half4 c3 = tex2D (_MainTex3, dbl_uv_MainTex0 - float2(1.0, 0.0));

            //this if statement assures that the input textures won't overlap
            if(IN.uv_MainTex0.x >= 0.5)
            {
                if(IN.uv_MainTex0.y <= 0.5)
                {
                    c0.rgb = c1.rgb = c2.rgb = 0;
                }
                else
                {
                    c0.rgb = c2.rgb = c3.rgb = 0;
                }
            }
            else
            {
                if(IN.uv_MainTex0.y <= 0.5)
                {
                    c0.rgb = c1.rgb = c3.rgb = 0;
                }
                else
                {
                    c1.rgb = c2.rgb = c3.rgb = 0;
                }
            }

            //sum the colors and the alpha, passing them to the Output Surface 'o'
            o.Albedo = c0.rgb + c1.rgb + c2.rgb + c3.rgb;
            o.Alpha = c0.a + c1.a + c2.a + c3.a ;
        }
        ENDCG
    }
    FallBack "Diffuse"
}