Hi Marilyn, I use combination fixtures that have a 250W MH and a 250W HPS in the same fixture. I also have one that has a 400W MH and a 400W HPS in the same fixture. Mine are about 5' above the plants, but no light mover. I would worry that 1000w would be too much at four feet or less and that you may be wasting energy or risk burning the plants. I would consider keeping the 600w and adding another 600 to expand your growing area.
John
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