When does flower start

biologically, it happens when you give the plant a long enough dark cycle. What you see comes 7-10 days later as it cannot manifest out of the ether. It has to grow. Week 4 is a bit early to judge. If it's a slower flowering plant, it can still add quite a bit, and even an 8 week variety isn't quite done packing on the weight, yet, either. if you plants are healthy and you don't do weird bro-science shit that delays flower, you probably just picked slower flowering plants as opposed to you causing it to be slow. Go look over old photos from same point in previous grows... it's a good reminder of expected development. (your grows. not others) YMMV

I'm sure you have just answered your own question my friend. Either give 2 weeks more or start counting 2 weeks later after the flip to 12/12... Hope this clarifies it for you.