I haven't done it with Blender EClark, but I have done it with Sculptris and with Z-brush. You can receive that error in either of those products as well. It can be caused by a couple of different things that I know of:

1. You have used a brush that does actually add or remove vertices, and that just won't import back into Poser.

2. More commonly, you are importing the wrong part of the body. Let's say we want to make our own lip morph. You export all of V4 to OBJ, import the whole model into Blender, make adjustments, and then export the whole model from Blender. Now you go to V4's head and try to import the morph. You're going to get that error, because it's trying to import V4's whole body into the head. So the number of vertices between her whole body and her head don't match.

What we should have done was export just her head from Blender. The other way I've gotten it to work is to import it into the Body as a FBM. This holds especially true If you are doing a morph that effects multiple body parts - you would have to export each part individually as an OBJ and then import them into that body part in Poser, or just do the whole thing in one go as a FBM.

Links I used when I tried this:

http://www.zbrushcentral.com/showthread.php?53850-Zbrush-Poser-Morphing-Tutorial

Hmm, that's the only I can find right now that I read before. There is this one that I haven't watched though that is specific to Blender:

http://preta3d.com/48/