four weeks into flowering and leaves yellowing.

My leaves are yellowing four weeks into flowering, i'm pretty sure its a nitrogen deficency. Should I add nitrogen at this point or let the leaves yellow and die?.

Thanks

Let them run their coarse IMO, it is the natural process of the plant drawing the stored carbs from the fan leaves to promote bud production and is a good thing. As the fan leaves die off the plant no longer needs to support their growth and can also contribute more energey to the bud development as well. Make sure you clear the dead fan leaves from the soil or they can become a breeding ground for bugs.
Adieu