I'm thinking more like 600 grams more or less. 4 lbs is kind of unbelievable. Are you doing an SOG set up? or fruiting all at once. Also how big are your plants going to be? If you're doing anything larger then small plants (1-2 feet finishing hight) you're going to need a lot more lightage than 1200w. You'd need something closer to 2000w. 1200 watts should get you a total of 30-40 small plants under both lights (15-20 under each) or 3-8 large plants under each light (once again depending on size). Realistically if you're working with large plants you need at least 200w of light per plant (i.e. 3 plants a light) to grow top quality. Also the larger the plant the more room you need to space them apart so that effects how many you can have under a light. If you're keeping them small you can stuff quite a bit under one area. I will say 70 sounds like too much, you need to remember that even small plants will grow 1.5 to 3 times their size fruiting. I'd do 30-40 small plants and you should get top quality bud or 3-6 large plants for top quality bud.
For 70-80 plants you should do 2 4x4 areas under each light with 2 1000w bulbs.
Generally when dealing with normal sized plants you get .5 to .75 grams per watt of light. A properly run SOG setups or a setup with lots of little plants like the one you may want to do generally get .5 to .75 oz per little plant. Smaller plants are a bit more productive since light penetration is better.
Even in an SOG setup the most you should put under each a 600w light is 15-20 small plants realistically. Anymore and you're just not maximizing you're use of light and molds can become an issue. The whole point of an SOG set up is use at lot of little plants and just enough of them so you can fully penetrate the canopy to develop each plant into 1 to 2 giant top colas. Thus you have a higher bud to leaf ratio increasing your overall yields since you'll have more smokable product. If you over stuff it you'll get a lot of tiny colas and some lower leafy/buddy mass below.
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