Here's the thing with the nitrogen in weeks 3 and 4. I use 1/2 bloom nutes, 1/2 veg nutes during weeks 3 and 4. At week 5 I will switch back to straight blume nutes and a bloom enhancer like A/N Overdrive which is high in P and contains no N. If I use that bloom enhancer on some strains without the extra N boost from earlier on, the leaves will go yellow and I'll lose my leaves.
I have found that my largest yeilds have come when I've avoided this problem by adding N in weeks 3 and 4. It's simple when you think that plants without leaves can't gather energy as efficiently.