why are there parts still under construction in 1-4? are these continued in 5 or just unavailable
Because Uberpie had a "brilliant" idea to break this down into 5 parts (currently) - with part 6 for the main route coming next - only in the last year, basically orphaning parts 1-4 at this time.
In order for the areas listed as under construction to be updated (or removed since they probably won't have any content for parts 1-5) Uberpie will actually need to focus for more than a month to actually update, edit, revise, fix whenever they get to breaking down those parts into separate downloads.
TL;DR - Because Uberpie doesn't have consistency in his work/vision and doesn't take the time (usually) to fix mistakes, errors, fuckups - which probably won't ever happen because fanboys won't stand waiting for lewd content simply so that Uberpie can fix anything. And no, they aren't continued in 5.