Yeah that's true too, if your right at the end/late flower then a couple leaves turning is natural, but if the whole plant becomes engulfed in yellow from the bottom up and you still have a few weeks left then I would def Nute one last time, and do a couple week flush. The plant is obviously going to use it's reserved energy in the leaves during the whole flowering process, however during the first 5 or 6 weeks you do want to makes sure there are plenty of Nutes available for the roots to uptake in the soil/medium. Don't be shy to give some Nitrogen, and if your only 4 weeks in or so you still have at least 4 or 5 weeks before the plant will be Fully ripe and "mature". I quotation the word mature because a wide majority of people consider the plant mature when red hairs are visual on the majority of the plant. You want to wait as long as possible as you gain plenty of weight during the end of the eighth week up to the 10th for indica's. Not only that the potency is gained as well. Unless you want a cloudy trich kind of high wait until their amber or until the plant generally looks done. Leaves will be curling into the bud and the hairs will be very red/orange and almost a gold looking. This time is the most optimal time to harvest as you will be harnessing all of the plants potential and the high will be awesome, and taste even better. So anyway yeah if your only half way done, than don't be shy to give some Nitrogen and other Nutes particularly P and K as well. So try to be patient, sounds like you have a ways to go, pics would be good to give a more accurate analysis of where your at.