Outdoor northern lights auto yellowing early

At 3-4 weeks of flower left, I usually don't worry too much about leaf discoloration personally. That all being said, at the end of flower, many plants have reduced nutrition needs and and start pulling resorces from the rest of the non flowering parts of the plants (i.e. the fan leaves), and it looks like that is what is happening here. If this plant truly does have 3-4 weeks. I would expect to aim at a harvest closer to 3 weeks as this just looks like this plant is super healthy and just coming to the end of it's life. I would suggest stopping feed in a week or two, or even just spacing out the feeding (i.e. if you are water feeding, instead of 2times a week, move to 1 time a week) and seeing how that works for you. With this many fan leaves at this stage of flower, It is honestly not uncommon for plants to do this. Best of luck! -DJ sunstone