If you installed V4 into your D|S runtime, and are trying to load it from there, that could be it. maybe, maybe not. You didn't say if it was plain old V4 or V 4.1 or V 4.2.  I had that problem with V 4.1 and apparently you have to install it into the Poser program directory. Why, I don't know, but if you haven't done that, give it a shot.
1.66 GB RAM should be enough as long as you don't overdo it on scenes, although it is kinda pushing it.