Just going by that pic, I'd say you have much more than 4 weeks to go. The last pic looks like some sativa I've grown. It may take even longer to finish than the others.
How about 12 more weeks at least... seriously. You should harvest when most of the pisitils(hairs) are orange or brownish.. you don't even seem to have any pistils just yet..
If we are extremely lucky there won't be a frost in September but October will see frost early for sure. I find that pot will mature not only by the amount of light but by temperature and other stresses too. Mother Nature looks after its creations. At least thats my story and I'm sticking to it...lol
Hey man Nice plants, what strain are they? You said you are going to harvest depending on how sticky your plants get. Why don't you look at the Trichromes and see what color they are. If they are clear don't harvest. If they are cloudy it will give you an uplifting high when you harvest. If they are cloudy/amber they will give you kind of a couchlock high. I harvest when half cloudy half amber and get the buzz that I want, I also check my Trichromes with a small Jewelers Magnifying Glass. They where all clear as per a couple days ago.
So what are trichomes ? Are they the small crystals on the pistals or the bulbous glands under the flowers ?