Dans le IShaderCallBack, ajoute :

    // set textures
    int a = 0; services->setPixelShaderConstant("Texture1", (const float*)&a, 1);
    a = 1; services->setPixelShaderConstant("Texture2", (const float*)&a, 1);
    a = 2; services->setPixelShaderConstant("Texture3", (const float*)&a, 1);
    a = 3; services->setPixelShaderConstant("Texture4", (const float*)&a, 1);

Et en GLSL :

uniform sampler2D Texture1;
uniform sampler2D Texture2;
uniform sampler2D Texture3;
uniform sampler2D Texture4;