Yellowing of the leaves under the main canopy?

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Its quite common to get a N difficiency in week 4 flower. I wouldnt give it too much attention because the "fault" already has been made. The plant uses so much nitrogen when its going into flower to stretch her arms that you wouldnt get it all green again in my experience. To solve this problem i used to not switch from grow into bloom nutes when i switched to 12/12 but i let the transition be smoother. and this is defenitely better with a stretching plant. For example the first week of 12/12 no bloom nutes at all. From the moment u see real budds forming u start the bloom nutes for like 1/4 of total feeding. and around end week 3 you should be at full bloom nutes. Good luck!

It is nitrogen deficiency and it is not a bad thing in flowering. Your plant wants more potassium and phosphorus as well. Likely more water. You should get run off when you water. I would add a light nitrogen in feeding, and a nice dose of potassium and phosphorus.

She looks hungry to me.