Shader Help: "Cannot implicitly convert 'float3' to 'float4'"

Hi,

I'm trying to experiment with using a shader to alter rendering based on the player position and a texture. I've taken a sample shader from the Unity docs and added a position vector input, as below:

   Shader "Custom/Test" {
     Properties {
       _MainTex ("Texture", 2D) = "white" {}
       _Detail ("Detail", 2D) = "gray" {}
       _PlayerPos ("PlayerPos", Vector ) = (0,0,0,1)
     }
     SubShader {
       Tags { "RenderType" = "Opaque" }
       CGPROGRAM
       #pragma debug
       #pragma surface surf Lambert
       struct Input {
           float2 uv_MainTex;
           float4 screenPos;
           float4 worldPos;
       };
       sampler2D _MainTex;
       sampler2D _Detail;
       float4 _PlayerPos;
       
       void surf (Input IN, inout SurfaceOutput o) {
           o.Albedo = tex2D (_MainTex, IN.uv_MainTex).rgb;
           float2 shadowUV = float2( IN.worldPos.x - _PlayerPos.x, IN.worldPos.z - _PlayerPos.z );
           shadowUV /= 32.0;
           shadowUV += float2(0.5,0.5);
 
           o.Albedo *= tex2D (_Detail, shadowUV).r;
           
       }
       ENDCG
     } 
     Fallback "Diffuse"
   }

The problem is that this fails to compile with the "Cannot implicitly convert 'float3' to 'float4'" error in the title being flagged against the last "o.Albedo *=" line although obviously there's generated code involved that I can't see.

As there are no float3 variables involved, and I'm expressly building a float2, I'm a bit lost as to what the issue is. Can anyone shed some light?

Additional: The issue seems to be to do with the shadowUV value as simply setting it to "shadowUV = float2(0.5,0.5);" allows the shader to compile and run (although not do anything of interest of course). I've tried passing the player position as two floats instead of via a Vector4 and it does the same thing.

Some more experimenting:

 float2 shadowUV = _PlayerPos.xz; //compiles
 float2 shadowUV = worldPos.xz; //throws the conversion compile error