So i'm a dick for not giving advice or making a judgment until there is useful information?4x4 room can take a 1000watt light if you have ventilation. Sorry some of these people jumped down your throat, lots of people are just dicks. 1/2 O isn't too bad under a 400. I personally think 1000 would give you some issues without some good ventilation, i would step up to the 600. They are the most efficient as far as lumens/watt plus it wont be as hard to keep temps in check.
Also you could add co2
More info may help...
strain?
time vegged ?
time flowered ?(when exactly do you decide to harvest and why ?)
seed or clone ?
Pics
an indication to ventalation..... heating...... distance light from plants etc etc.....
I think this should really helpHow long has this problem been going on?
What system are you running? (DWC? Ebb flow? Aero? Water Farm? Flood Tables? and so on...)
What STRAIN are you growing?
What was the establishing technique? (Were the seed or clone?)
What is the age of your plants?
How tall are the plants?
What PHASE are the plants in? (seedling, vegetative or flower) are the plants in?
What Technique are you using?
What substrate/medium are you using?(Hydroton, RockWool etc.)
What is the Water temperature?
What color are your roots? White? Brown? Are your roots slimy?
What Nutrient's are you using? How much of each if using multiple?
What is the TDS/EC/PPM you are using?
What is the pH of the "Tank"?
Are you sure your calibration is correct on your equiptment?
When was your last watering?
What is your water temps?
When was your last feeding change? (ie. grow-bloom-micro-additional)
How often do you clean your system: example: Flush out water replace with clean water and nutrients?
What size bulb are you using?
What is the distance to the canopy?
What is your RH Factor(Relative Humidity)?
What is the canopy temperature?
What is the Day/Night Temp? (Include flucutaion range)
What is the current Air Flow? (cfm etc.)
Is the fan blowing directly at plants?
Is your water HARD or SOFT?
What water are you using? Reverse Osmosis (RO)? Tap? Bottled? Well water? Distilled? Mineral Water?
Has plant been recently pruned, cloned off of or pinched
Have any pest chemicals been used? If so, What and When?
Are plant's infected with pest's