The P4 rendering engine still has uses, even in P6.  I rarely use it for close up work unless it is something I intend to do a lot of postwork on, but right there, why spend time waiting for Firefly to render?  If I'm doing a backgroudn to comp in later and there are no reflections or displacement, they I use P4, again, why wait for Firefly?

I also often use P4 as a reference render that I save to a tif and use for the alpha channel, this is if I render a complex scene, but then want to isolate the characer(s) or props or some such for post work.  I just go delete what isn't needed in the alpha channel and do a P4 render.

But, why anyone would want to stick to Poser 4 or Poser Artist is beyond me.  Really, Poser 5 is available cheap and the improvements are well worth it even if you are going to render only in P4 mode.

Oh, and Daz has announced they no longer will be supporting P4 on their new items.  It's just time.