We went through this a few months back, (I think it was a thread at 3DCommune) and I had contacted SM at that time.

Their reply stated something to the effect that their purchase agreement for Poser did not allow them to distribute the Poser-4.03 update, and they had no real suggestions on where it might be obtained. 

As mentioned above, there seems to be no authorized distribution outlet for that update  - but, look around - it has been periodically re-posted by other individual users.