mate ur plants are fine and at 4 weeks into flowering i wouldn't be giving them any N . N is only used for veg growth and doesn't make any more bud . ur plants are taking the N from the leaves , this is totally normal all plants do this as the get to the end of their life cycle . TAKE IT FROM ME UR PLANTS DON@T NEED N . if u start feeding N now u'll leave ur bud harsh to smoke . continue with ur flowering feed for another 2 weeks them flush them and give them just ph adjusted water for the last 14 days.