hborre is a lot smarter than  I;  and his answer is, of course, correct.  But just in  case you  are not familiar  with Poser..... here is a little detail ( I do  what  you  are describing all the time)

1. load V4 into  a new Poser scene.
2. just  as a good habit to  get into,  go : WINDOW- JOINT EDITOR-ZERO FIGURE
   3. go: FILE- EXPORT- ( choose the format you want - The best  choice is wavefront OBJ)
    
3. single frame will  be checked ,click  to accept single frame
4. you will  have now an "item list"  ....uncheck  EVERYTHING  except for the item "head" -click  OK
5. you  will see "export options"-  these choices are someewhat  subjective,   but checking less options  is probably better...since you  are specifically morphing  only a head, I would uncheck  everything, you  might want to  leave the checkmark on "include body part names......."  because it is a head and you  can slect  lashes, lips, etc  easily in most modeling programs when the names of all of these things are intact.

7.name and save the head. Use this model  in your modeling program. You  cannot change the poly count, poly order.....   poser will not reorder well even though it has an option  to  try and fix the order.
....and what  is thew biggest  PAIN OF ALL, IMO, is you  cannot split the mesh in  half, model it, then  mirror the half.  That  will  mess up the polys  for Poser.

anyways,  when  you  are done you will want to save as wavefront OBJ.

8. go  back  to  poser and in pose room, select the body  part head.
9. on properties tab of  parameters window, click  load morph target. choose your saved OBJ from your modeling program.

a morph dial will appear , named the same as your OBJ,  in the parameters  tab, under "Morph"

10.  crank the morph dial; enjoy morph.

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