SplatMaps and packing variables

Hello folks,

I am currently trying to wrap my head around terrain texturing using splatmaps. As a base I a use this shader which I found on reddit :

 Shader "Custom/Splatmap" {

 Properties {
 _MainTex ("Splat Map", 2D) = "white" {}
 _Texture1 ("Texture 1", 2D) = "white" {}
 _Texture2 ("Texture 2", 2D) = "white" {}
 _Texture3 ("Texture 3", 2D) = "white" {}
 _Texture4 ("Texture 4", 2D) = "white" {}
 }

 SubShader {

 Tags {"RenderType" = "Opaque"}


 CGPROGRAM
 #pragma surface surf Lambert


 struct Input {
         float2 uv_MainTex;
         float2 uv_Texture1;
         float2 uv_Texture2;
         float2 uv_Texture3;
         float2 uv_Texture4;
 };


 sampler2D _MainTex;
 sampler2D _Texture1;
 sampler2D _Texture2;
 sampler2D _Texture3;
 sampler2D _Texture4;


 void surf (Input i, inout SurfaceOutput o) {        
         half4 splatmapMask = tex2D (_MainTex, i.uv_MainTex);
         half4 combinedColor = tex2D (_Texture1, i.uv_Texture1) * splatmapMask.r;
         combinedColor += tex2D (_Texture2, i.uv_Texture2) * splatmapMask.g;
         combinedColor += tex2D (_Texture3, i.uv_Texture3) * splatmapMask.b;
         combinedColor += tex2D (_Texture4, i.uv_Texture4) * splatmapMask.a;

         o.Albedo = combinedColor;
         }
 ENDCG
  }



  FallBack "Diffuse"
  }

The first problem is, this shader doesn't work as it is, I have to remove one of the textures, else I get " Too many texture interpolators would be used for ForwardBase pass (11 out of max 10)"

So I read and read and found I have to add

  #pragma target 3.0

but still I have to remove a texture to make it work.

So I read further and they said I have to pack together my input variables, so 2 float2 become one float4, but here my lack of shader programming skill comes into play... I am not sure how to do that.

I'd imagine to do it like this :

    struct Input {
      float2 uv_MainTex;
      float4 uv_Texture1and2;
      float4 uv_Texture3and4;
    };

but how do I get them apart again in the surf function? And I am not even sure where this input comes from and how it knows it contains 2 texture coords in 1 variable.

Please, if somebody could enlighten me... Bonus question, how are the 11 interpolators calculated, where do they come from?

Thanks