$lumblendfactor2

$lumblendfactor2, $lumblendfactor3, & $lumblendfactor4 allow you to choose which luminance value to use for blending in Lightmapped_4WayBlend materials.

This is choosing between the luminance of the given layer and the inverse of the blended luminance of the layers below it using a lerp.

lerp( 1 - blended_lum_of_layers_below, lum_of_current_layer, lumblendfactor )