You have different methods of doing it. You can either export the mesh into a 3rd party program (i.e., Blender, etc.) and tweak it until you get something satisfactory or spin dials in Poser for each facial part. The morphing brush can be a great tool for manipulating the mesh within Poser but you need to develop a delicate touch. Face Shop is another alternative but considering that you still gravitate to a P4 model for your renders, it's pointless to recommend a software heavily invested in Genesis and La Femme and L'Homme.