Rigging is a functional part of the model, so it is likely not subject to copyright.  However, as Richard points out, using the rig without requiring V4 would almost certainly violate the EULA. 

However, Poser Place Outfitter (available free at RDNA) might be the basis of a script to transfer used by the user to transfer the joint setup from V4 owned by the purchaser to your model (I haven't looked at it in ages, so I'm not certain it works with non weight-mapped rig, but I think it may, as V4WM retains the classic sphere joints in the hands and feet).  Outfitter is open-source under the GPL.

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