get the lights as close as possible without burning the plants, 600 watters should be no more than 18 inches above the plants, the closer the plant = more light= more bud, place your hand on the top of the plant, palm down, then move the light closer until you just start to feel the heat on the back of your hand, if its comfortable for you its ok for them, just make sure you alow room for your girls to grow over night without them burning, 13-14 inches should be good, also have you got an oscillation fan blowing across the tops of them? if not the get one, this will allow you to get the light closer + strengthening the plants also improving yield. also with 4x600 you should pull 2 oz a plant easy! what size were they vegged to? also your at the right stage of flowering to add a pk booster, i use pk 13/14 from week 4ish to week 6, makes a big difference. imo
p.s on your next grow veg till at least 18 inches which is about 4 weeks grow time and you should pull about 4oz a plant.