Is mesh.uv4 used for or going to be used for anything?

I know uv2 is used for lightmapping and uv3 is also used by the standard Unity engine in some cases. Is mesh.uv4 used by any Unity effects?

Edit:

Also, I saw this in the shader files: [Enum(UV0,0,UV1,1)] _UVSec ("UV Set for secondary textures", Float) = 0

Does this refer to UV2 or the undocumented UV1? Also, is UV1 depreciated?

Edit:

Answer for two of my questions: UV1 in the shader actually refers to mesh.UV2. This is used for lightmapping, but also in Unity3D for detail mapping (unless UV0 is specified). Additionally, I found this in UnityCG.cginc:

 // Note: V2F_SHADOW_COLLECTOR and TRANSFER_SHADOW_COLLECTOR are deprecated
 #define V2F_SHADOW_COLLECTOR float4 pos : SV_POSITION; float3 _ShadowCoord0 : TEXCOORD0; float3 _ShadowCoord1 : TEXCOORD1; float3 _ShadowCoord2 : TEXCOORD2; float3 _ShadowCoord3 : TEXCOORD3; float4 _WorldPosViewZ : TEXCOORD4

It also uses TEXCOORD4 which is only defined for XBox:

 #if defined(SHADER_API_XBOX360)
     half4 texcoord4 : TEXCOORD4;
     half4 texcoord5 : TEXCOORD5;
 #endif

And is depreciated (not sure why). I'm not sure if there's any non-depreciated use of Texcoord3.