Widow,

I need a little more details on your last watering.

What was the concentration of nutes you mixed? are you over watering? usually when you over nute or water the leaves turn yellow starting at the tips not shrivel up,

Heat what is that at in your grow room, my experiance is when the leaves shrivel up it is due to to much heat or under watering.

Also you are in 4th week and plants are 12" high. Time to go to flowering cycle and nutes, I would wait on nutes untill we figure out why your plants are sick.