Unknown input semantics TEXCOORD/4

I've set

 #pragma target 3.0

in the shader, but I cant get TEXCOORD4 to work. I get the following error when I add it to my shader. Its commented out in the code below, with or without doesn't work. Is TEXCOORD4 allowed as an input? Its happy with it in just the v2f struct.

 Shader error in 'Custom/TileRenderer': Vertex program 'vert': unknown input semantics TEXCOORD/4

What am I doing wrong?

 Shader "Custom/TileRenderer" {
 Properties {
     _MainTex ("Base (RGB)", 2D) = "white" {}
     _AlphaTex("Trans (A)", 2D) = "white" {}
     _OverlayTex("Trans (A)", 2D) = "white" {}
 }
 
 SubShader {
     Tags {"Queue"="Transparent" "IgnoreProjector"="True" "RenderType"="Transparent"}
     LOD 100
     
     ZWrite Off
     Blend SrcAlpha OneMinusSrcAlpha 
     
     Pass {  
         CGPROGRAM
             #pragma vertex vert
             #pragma fragment frag
             //#pragma target 3.0
             //#pragma multi_compile_fog
             #pragma debug
             
             #include "UnityCG.cginc"
 
             struct appdata_t {
                 float4 vertex : POSITION;
                 float2 texcoord : TEXCOORD0;
                 float2 texcoord1 : TEXCOORD1;
                 float2 texcoord2 : TEXCOORD2;
                 float2 texcoord3 : TEXCOORD3;
 
                 //as soon as I add TEXCOORD4 in the appdata_t struct
                 //I get the shader compile error
                 float2 texcoord4 : TEXCOORD4;
 
                 //UNITY_VERTEX_INPUT_INSTANCE_ID
             };
             
             struct v2f {
                 float4 vertex : SV_POSITION;
                 float2 texcoord : TEXCOORD0;
                 float2 texcoord1 : TEXCOORD1;
                 float2 texcoord2 : TEXCOORD2;
                 float2 texcoord3 : TEXCOORD3;
                 float2 texcoord4 : TEXCOORD4;
             };