Textures are blended by thier alpha channel, just give the terrain 4 skins, and and then make sure youn define the detail texture somewhere and assign it to material skin 1.

Code:

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//fixed function terrain shader, blends 4 color texture based on thier alpha channels,
// and blends a detail texture over that


////////////////////////////////////////////////////
// Assign the tile size of the Textures.
//

float Skin_Size1 = 60;
float Skin_Size2 = 60;
float Skin_Size3 = 50;
float Skin_Size4 = 40;

//detail texture
float Skin_Size5 = 400;

float4 vecLight;


/////////////////////////////////////////////////////////////////////////////////////
// Let's allow for adjusting the intensity of the ambient light color on the passes.
//
float ambientLightFactor = 0.5;


////////////////////////////////////////////////////////////////////
// Here we just create and plug some values into a matrix for
// use in multiplying during the stages.
//

float4x4 matTerrainTexSize =    {1.0, 0.0, 0.0 ,0.0,   
		    		 0.0, 1.0, 0.0, 0.0,   
		   		 0.0, 0.0, 0.0, 0.0,
		    		 0.0, 0.0, 0.0, 0.0
				};


float4x4 matAlphaMapSize = 	{1.0, 0.0, 0.0 ,0.0,
		    		 0.0, 1.0, 0.0, 0.0,
		    		 0.0, 0.0, 0.0, 0.0,
		    		 0.0, 0.0, 0.0, 0.0
				};


Texture mtlSkin1;	//detail texture
Texture entSkin1; 	//1st tile tex RGB
Texture entSkin2;	//2nd tile tex RGB and blender in A
Texture entSkin3;	//3nd tile tex RGB and blender in A
Texture entSkin4;	//4nd tile tex RGB and blender in A


sampler s_ShaleTex = sampler_state
{
    Texture = (entSkin1);
    MipFilter = Linear;
    MinFilter = Linear;
    MagFilter = Linear;
    AddressU  = Wrap;
    AddressV  = Wrap;
    AddressW  = Wrap;

    MipMapLodBias=-0.2;
};


sampler s_SandTex = sampler_state
{
    Texture = (entSkin2);
    MipFilter = Linear;
    MinFilter = Linear;
    MagFilter = Linear;
    AddressU  = Wrap;
    AddressV  = Wrap;
    AddressW  = Wrap;

    MipMapLodBias=-0.2;
};


sampler s_GrassTex = sampler_state
{
    Texture = (entSkin3);
    MipFilter = Linear;
    MinFilter = Linear;
    MagFilter = Linear;
    AddressU  = Wrap;
    AddressV  = Wrap;
    AddressW  = Wrap;
 
    MipMapLodBias=-0.1;
};


sampler s_RoadTex = sampler_state
{
    Texture = (entSkin4);
    MipFilter = Linear;
    MinFilter = Linear;
    MagFilter = Linear;
    AddressU  = Wrap;
    AddressV  = Wrap;
    AddressW  = Wrap;

    MipMapLodBias=0;
};

sampler grass_detail = sampler_state
{
    Texture = (mtlSkin1);
    MipFilter = Linear;
    MinFilter = Linear;
    MagFilter = Linear;
    AddressU  = Wrap;
    AddressV  = Wrap;
    AddressW  = Wrap;
    MipMapLodBias=0;
};


	
technique technique_one

  {
   	
    pass p0
      {
	Sampler[0]	= (s_ShaleTex);
	
	AlphaBlendEnable = False;
	AlphaTestEnable = False;
	DitherEnable = True;
	zWriteEnable = True;
   zEnable = True;
	Lighting=True;		
	
	Ambient = mul(vecLight, ambientLightFactor);
	
	TextureTransformFlags[0] = Count2;
	TexCoordIndex[0] = 1;
	TextureTransform[0]= mul(matTerrainTexSize,Skin_Size1);
						
	ColorOp[0] = Modulate2x;	
	ColorArg1[0] = Texture;
	ColorArg2[0] = Diffuse;
	AlphaOp[0] = Disable;
	
     
      }


    pass p1 
      {
	Sampler[0] 	= (s_SandTex);   	// Blendmap is Alpha
	Sampler[1] 	= (s_SandTex);     	// The Colormap 
	
	Lighting = True;
	         		
	AlphaBlendEnable = True;

	SrcBlend = SrcAlpha;
	DestBlend = InvSrcAlpha;

	zWriteEnable = True;
   zEnable = True;	
	
	Ambient = mul(vecLight, ambientLightFactor);

	TextureTransformFlags[0] = Count2;		
	TextureTransform[0]= (matAlphaMapSize);
	texcoordindex[0]=0;
	ColorArg1[0] = Texture; 
   ColorOp[0] = SelectArg1;
   AlphaArg1[0] = Texture;
   AlphaOp[0] = SelectArg1;

	TextureTransformFlags[1] = Count2;		
	TextureTransform[1]= mul(matTerrainTexSize, Skin_Size2);
	texcoordindex[1]=1;

	ColorArg1[1] = Texture; 
	ColorArg2[1]= Diffuse;
   ColorOp[1] =Modulate2x;
   alphaarg1[1] = current;
   alphaop[1]=Disable;

      }

    pass p2
      {
	Sampler[0] 	= (s_GrassTex);   	// Blendmap is Alpha
	Sampler[1] 	= (s_GrassTex);     	// The Colormap 
		
	DitherEnable=True;
		
	AlphaBlendEnable = True;

	SrcBlend = SrcAlpha;
	DestBlend = InvSrcAlpha;

	DitherEnable = True;
	zWriteEnable = True;
   zEnable = True;

	Ambient = mul(vecLight, ambientLightFactor);

	TextureTransformFlags[0] = Count2;		
	TextureTransform[0]= (matAlphaMapSize);
	texcoordindex[0]=0;
	ColorArg1[0] = Texture; 
   ColorOp[0] = SelectArg1;
   AlphaArg1[0] = Texture;
   AlphaOp[0] = SelectArg1;

	TextureTransformFlags[1] = Count2;		
	TextureTransform[1]= mul(matTerrainTexSize, Skin_Size3);
	texcoordindex[1]=1;

	ColorArg1[1] = Texture; 
	ColorArg2[1]= Diffuse;
   ColorOp[1] =Modulate2x;
   alphaarg1[1] = current;
   alphaop[1]=Disable;

    }

    pass p3
      {
	Sampler[0] 	= (s_RoadTex);   	// Blendmap is Alpha
	Sampler[1] 	= (s_RoadTex);     	// The Colormap 
	
	Lighting = True;
				
	AlphaBlendEnable = True;

	SrcBlend = srcAlpha;
	DestBlend = InvSrcAlpha;
	
	Ambient = mul(vecLight, ambientLightFactor);

	TextureTransformFlags[0] = Count2;		
	TextureTransform[0]= (matAlphaMapSize);
	texcoordindex[0]=0;
	ColorArg1[0] = Texture; 
   ColorOp[0] = SelectArg1;
   AlphaArg1[0] = Texture;
   AlphaOp[0] = SelectArg1;

	TextureTransformFlags[1] = Count2;		
	TextureTransform[1]= mul(matTerrainTexSize, Skin_Size4);
	texcoordindex[1]=1;

	ColorArg1[1] = Texture; 
	ColorArg2[1]= Diffuse;
   ColorOp[1] =Modulate2x;
   alphaarg1[1] = current;
   alphaop[1]=Disable;

      }
     
    //detail texture pass 
    pass p4
      {
	Sampler[0] 	= (grass_detail);   	// Blendmap is Alpha
	Sampler[1] 	= (grass_detail);		
	AlphaBlendEnable = true;
   fogcolor={1.0,1.0,1.0,1.0};
   fogstart = 100.0f;
   fogend = 500.0f;
	Ambient = mul(vecLight, ambientLightFactor);
	
	//multiply blending
	srcblend=zero;
	destblend=srccolor;
	lighting=false;

	zWriteEnable = True;
   zEnable = True;		
	
	TextureTransformFlags[1] = Count2;		
	TextureTransform[1]= mul(matTerrainTexSize, Skin_Size5);
	texcoordindex[1]=1;

	ColorOp[1] = modulate;	
	ColorArg1[1] = Texture;
	ColorArg2[1] = diffuse;
	AlphaOp[0] = Disable;

      }
   
 }
   

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