Where I can I get a 48in 4bulb t8 light fixture?

No expert on lighting but 4 48" bulbs over 55 gal tank will be extremely high lighting

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Over a 20-ish inch tall tank it wont be too much IMO...however, you could possibly same some of the hassle of trying to find/fit a fixture by maybe going the dual T5NO route or finding a T5HO that can be either raised or runs bulbs individually.
 
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