I’m not sure what the diagnosis is, but I can tell you part of the problem is no nutes added and you’re in the 4th week of flower.
This is from the Biobizz website:
This heavily pre-fertilized substrate mixture emulates a rich outdoor soil with a full micro active ecosystem. It’s been designed to have enough nutrient-fuelled power to sustain lush growth for a couple of weeks, without any need to add any extra fertilizer.