I've never used HID so its hard to say how it would compare exactly, but two in that size area would be about as good as you can get for lighting. I have one DS400 and plan on getting two more to replace some of the DS100's and Blackstar's that I have. The buds under the 400 are always much fatter--granted it is in the center, but even when run alone it does better than the others. My DS100's were built by eShine before Advanced dumped them as a manufacturer due to quality problems and they do seem to fall a little short when comparing 4 of them to the DS400. Plus having that many lights is annoying to manage.
What I like about the DS400 is that it is ideal for a 4' wide section. Its long and rectangular with a 4x2.5' coverage area, so you can get good overlap in the center with your 4x4'. I don't think you can get a single LED that will cover the area with as good as two of these. Other light fixtures have square or rectangular footprints, but often don't square up when combining them, usually they will be for an area not the exact size as yours. I.e working with a 2x3' footprint fixture would be more difficult to get even lighting. You could do it with four 2x2' fixtures but then your dealing with four fixtures--harder to hang in a tent with only a few poles, they slide around and are a bitch to work with.
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@Phychotron.....delicious looking specimens!!!