From what i can tell, it looks like a potassium deficiency. Of course that's without seeing the rest of your plant.
If your lights are HID's the plants under those lights can use more food than plants under floros.
4-3-6 seems really week. But your plants could be smaller than they look or sativa dominant...so there's that.
to get a better diagnosis, please fill out one of the forms that are stickied in this forum. light wattage, plants size, sativa or indica dominant?
these questions and many more are the key to getting a lock on the problem. you have growers on here with years of experience that know
how different variables effect plants. More info is always better when it comes to diagnosing issues, even if you dont think it matters.

Hope this helps...
GanjaRobPDX Reviewed by GanjaRobPDX on . brown spots, some twisted leaves and some tips curled down I'm honestly completely stuck at what the problem could be with my plant. It's been flowering for maybe a month or more. It started getting brown spots all over the leaves (but only some leaves). They're little speckles and over the whole leaf, not just the tips. They're not yellowing either, only a couple of the leaves have actually died and those leaves turn completely brown/black and yellow from the tip. Also, some of the tips of the leaves curl down but then other leaves are twisted. None Rating: 5