Fan leaves turning 4 weeks into flower

I would be continuing/re-introducing with some half strength grow nutrients every second feed, she is suffering nitrogen deficiency. Your problem is common amongst people who switch to straight bloom nutrients too early, cannabis still needs adequate nitrogen during early floral development. Personally I continue with grow nutrients for the first 2 weeks of flowering before changing to half grow and half bloom for the next 2-3 weeks before then changing again to just straight bloom nutrients through until maturity. This seems to produce the biggest blooms on plants that don't exhaust themselves and is a method I have been using for 25+ years and I swear by its effectiveness. Hope this helps, Organoman.