I know 4 is more than twice as good as 2 bulbs. But since you only have 4 Phals and I'm SURE you won't be getting any more, perhaps you should only get a 2 bulb 2 foot fixture. 6500K is the best. I only use 4 foot fixtures and either 4 or 6 bulbs. I can't tell you how many hours because I'm using T8s right now. 4 T8s are 128 watts vs 4 T5s are 208 watts !! BIG difference there ! I have mine all on a timer and they're on 15 hours/day during DST and 12/day when off.
HTH,
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