If the theories that Harry is the final one are true, perhaps that would unintentionally (for Voldemort) complete the pattern. Dumbledore could always be wrong about where the 4 remaining ones are, so that it really is one from each house. I can't remember how it all breaks down exactly now. One's in Voldemort, one's the locket, one's possibly the cup...there's got to be some way of getting all four houses in. Maybe one horcrux is something from two houses together...?