Sorry, I didn't mean to scare you, you've been asking what's going on - so - I gave you my thought about your chlorine demand.
I think that 4ppm FC is too low and you need to add some more bleach or Liquid Chlorine (LC) sooner than later.
If you don't see or feel any algae then let's stay with regular FC (5ppm - 10ppm) levels for a while.