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...