@ 4 feet then I would say 400watts would be an abslolute minimum! & 1000 watt would be adequate i guess. But nothing really compares to the sun. It's very hard to match that light indoors.
So have you already pulled this plant up? Where do you live (what state & how far north or south), basically I'm wondering what the weather is like there now, obviously i don't want you to tell your address
. My point is if your plant has been outdoors then it should be already budding & digging up an oudoor plant & moving it indoors can be difficult if not impossible to do without causing alot of stress to the plant. So if possible I would say leave your plant outdoors & let it continue to flower unless the forecast calls for temps dipping below freezing. That being said I would want to be ready w/ whatever you need if the plant looks like it's that far off that it's not gonna be ready for quite a while & the weather is already flirting w/ those temps.
Basically you need to weigh in the stress of letting the plant mature outdoors & how bad you think that will be vs. the stress of moving the plant indoors, the root dmg, the cost of the indoor lighting & set-up, & the production hit you could take from having inadequate light on a large plant if you don't spend alot of $$$$ on alot of lights which the plant would need. Now if you spend asot of money on lights & you plan on using them regularly & more than for just this 1 project... well then that could factor into making the lights a better buy. Anyhow I hope you see my point by now.
Lots of things come into play. Are you growing this plant for strictly personal use or are you hoping to make some $$$ from it? If you want to make $, then are the lights you are going to need gonna be worth it? If you skimp on the lights how much weight is it gonna cost you? If you leave it outside & it gets hit w/ frost how much quantity will that cost you? How much quality?(again wether it's for personal use or for profit may come into play on that one).
Anyhow, I can't really answer those ?'s for ya. Something to consider & it's a tough decision. I just hope that you are as informed as you can be b4 you make a decision to pull a healthy plant from the earth. It's a big decision to make & since it's been outdoors this long I would not want to pull it unless I had to.
P.S. Oh, it just occured to me that this may be outdoor & potted, in which case it's not as big an ordeal as I've been thinking. You could keep an eye on the weather & if temps are going to drop below freezing overnight then you could bring it in. If you don't need to bring it in permanent then that would make it easier to skimp on the lighting. I would say 400 minimum still though. 600 would be better. Leave it out if it's potted & if it's gonna drop below freeze, bring it in & keep your light timed to come on 5 min after sunrise & go off 5 min before sunset. If the weather warms up then set it back outside. in fact I would say, same goes even if it wasn't potted. I mean the part about putting it back under the sun as long as the weather warms back up. & keep your lights in tune w/ the sunrise/sunset if you are going to do this. Good luck!
