Here we go again:
You're being sold a bill of goods by somebody that's not telling you the whole story. People try to make things sound like they're automatic: You buy this and that and you'll get X.
With some simple topping under a 400 watt HPS, you can easily get 2.5 to 3 ounces per plant. If you expect to get 5 ounces per plant or more, you're going to pretty much have to scrog it, which is a whole lot more work.
Also, don't think of it as watts per plant. You're talking about an area, not a plant. A 400 watt HPS will cover a 4 x 4 area pretty well. That's why if you get the 4 x 4 tent, the 400 HPS is all you need to fill it with light. Would a 600 do a better job? Well, yea, but only slightly more so. You wouldn't notice a huge difference in plant growth, but you would notice a difference in bud growth and quality.
So if I were you, and were going to grow in a tent, I would get the 4 x 4 no matter what light you decide to go with. The 400 you already have will do just fine in it. A 600 would be slightly better, but again, don't think it's a night and day difference. It isn't.
Do a LOT of reading. You have a lot of folks who tell you a lot of numbers without explaining how they go about getting them. It causes a hell of a lot of confusion, especially when someone goes out spending tons of money, don't get those type of yields, and can't figure out why.
