I'm currently using DS3 Advance and don't have DS4 yet because I'm waiting for the 64bit version (and hopefully a better GUI, most likely as a plugin/addon).

If you are using DS4 32bit version to render then it cannot access more than 3.2GB of RAM so that maybe why the render is taking so long. Any 32bit program (even if run on a 64bit system) cannot access more than 4GB of RAM (and Windows need approx 0.8GB of RAM to run in the background). This is because of the original design limits of the 32bit system the program was made for.

Another reason renders in DS (all versions) take a long time to render is because of shadows rendering through transperancy maps in hair and eyelashes. This doesn't seem to cause an issue to Poser but to DS it does cause really long render times. I remember reading a walk around for this in Daz forum so check there for further info.