When you say problems, I am assuming you mean the browning on the lower leaves, my money would be on nutrient spillage but I am a hell of a long way from being an expert.
To my mind, the newer growth looks lreal good and overall you have some lovely looking plants.
I would recommend reading this and bookmarking it for future reference, I have found it invaluable
http://www.marijuanapassion.com/forum/showthread.php?t=53717&page=2
As for flowering, you have 16 sq ft in your 4x4, burning 1800W (I am assuming you have 3x 600W HPS) is overkill!
It is normally recommended you use a min of 5,000 Lumens per sq ft during flowering. A 600W will give you around 95,000 lumens so two of your 600w lights will give you 190,000 lumens which is just short of 12,000 lumens per sq ft. You could actually just about run 1x 600W in that space.
I am saying this to try and save you a few $$$$$ on your electric bill.
Your girls could triple in height during flowering so I would suggest taking your max headroom, deduct height of pots, size of lamps and say 12" for distance from lamps to light. This gives you the max plant height. Divide that by 3, giving you a safe height for starting flowering.
If you grow this strain again, you will have an idea of how much they will stretch during flowering but for now that is their safe height (without LST).
Now, that is the max you can go to but it is up to you when you switch to flowering. If they are 6 weeks old, they should be sexually mature but I could not tell from the vid if they had alternating nodes.
So, the short answer to your question is, any time between now and the height you worked out!
And.. The browning doesnt look anything serious to me but I am sure someone with more experience will let you know if I am wrong.
Sorry for being long winded but I believe if you know the whole answer to something, it helps you work out this stuff in future.
This is mostly IMHO, though some of it is fact.
Welcome aboard dude and green mojo on its way.
Stay frosty. W
