From the link
Nitrogen (N) Mobile Element and Macro Element
Benefit: Nitrogen plays a very big role in your plants; this one element is directly responsible for production of chlorophyll, photosynthesis, Amino Acids, which are the building block of Proteins. The myriad of enzymes which help the plants growth in leaves stems and the how well the vigor of your plants is.
Nitrogen is the biggest mobile element meaning it can travel anywhere on the plant.
Usually the def will start on the lower to middle part of the plant, and then will usually happen to older leaves first. Then the deficiency will work its way up the plant. Your plant can be green on top, then yellowing on the lower leaves when the deficiency is starting out. Yield will be greatly reduced without good amounts of nitrogen in your plants. Sometimes in bad cases the leaves will turn a purplish color along with the yellowing.
Unlike a magnesium deficiency, nitrogen def will start from the tips and work its way back to the leaf node. Nitrogen and Magnesium get confused. The best way to tell them apart is, nitrogen deficiency starts around the tips and works its way to the back of the leaves, where a magnesium deficiency will cover the entire outer part of the leave and make the entire leaves yellow leaving the veins to stay green. If your plants are having a slow growth rate and have yellowing of the leaves, then most likely it’s a nitrogen deficiency.
Towards the middle to end of flowering stages, the plant will show a nitrogen deficiency almost always. This process is completely normal and just let the plant naturally yellow out as it uses it's stored nutrients. This actually helps you by getting ready for final flushing and then harvesting. At this point DO NOT not use nitrogen to fix the problem. The yellowing leaves will then eventually drop off after the plant is done with them.
4 weeks is not to early imho.