A lot of rendering engines (ever true ray-tracers like POV-Ray) have trouble applying highlights to anything that's 100% black. Actually, what it is in this case is that when you 'apply texture to highlight' you're essentially using a specularity map, it's just that in P4 you can't use a different specularity map than your texture has. So, since you have black spots on the texture, and that texture is being used as a specularity map, those sections have a black highlight, which doesn't highlight. It can be hard to tell, though, because of the way highlights show up in Poser. In other words, basically, in P4 using 'apply testure to highlight' is the same thing as, in P5, using the same map for both diffuse colour, specular colour, and specular strength (though not having P5 I don't remember what those nodes are actualy called off the bat, so I'm using bastardised Max names)