so I take it that you are dripping?

funny thing...when you research you find that plants can wilt from overwatering/underwatering, and that kinda makes you scratch your head! lol

watering 15 min out of every 4 seems light, but if plant perks up when light are on and droop otherwise...that seems like overwater...
what size rockwool are you using?

On another note; The only time that my palnts droop is from lack of nutes/oxygen, so that would make you think of overwater...you know Z will clear this up when he comes around, but I would like to help if i can, until we get further confirmation...and pix would help greatly...I know how you feel when your babies don't look so good...