The pics aren't super clear. I would suggest cutting back your nutes. Your plants look like they might be a little over ferted there. Can you take some pics with the light off?
It just looks like you are getting the "clawing" effect on your leaf tips. What kind of light are you using?
What is the temp at the top?
Any chance you know the humidity?
Oh, I left this thread and immediately the next thread I saw was this one...
https://www.rollitup.org/marijuana-plant-problems/162605-leaves-dying-flowering-pic.html
So to answer your question, THAT's why it says to use Grow Big during week 3 and 4.