400vs600

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    24x30inch area accompanied by 4 plants 2 weeks into flower - 400w vs 600w?

    My grow area is 24 inches deep and 30 inches wide and there are 4 plants 2 weeks into flower... I have a 600 watt dimmable ballast and I have the ballast set at 400watts.... someone told me that you only need so much light and it becomes a waste after too much but do you think that a 24 by 30...