There is a bug happening in cc4 when exporting head mesh for metahuman it doesn't allow the "high" quality option in the drop-down selection. it says for the high option you need to bake the textures at 4k in the skin editor but no matter if you do that or even select the stock character base meshes that say 4k it still doesn't allow it. It seems to work with one of the cc3+ character meshes but it doesn't work on most skingen examples that are supposed to be high quality.