Anyway, I was thinking that putting a Part from the neighbors' perspective is 'interesting' but unwanted; it would take away from the 'mystery' element.
So instead, how about basing it all around how those neighbors live and how they look in the eyes of others. So that Part 4 could be (a) grand scene(s) where many people act on their interest in these neighbors... Maybe make the police show up? What if they were CIA sleepers living in another country or maybe they're a couple (2) of mad scientists?