This fixture, according to their website, has 2 ballasts and 2 switches. You must fully populate all the lights on the ballast you are using. This means you either run 4 or 8 bulbs. This fixture would be appropriate on a 120 gallon (4x2x2) marine tank with 4 10,000K and 4 actinic bulbs.
Their 4x54 is on sale for 190. With this fixture you can run 2 or 4 bulbs. You can request the bulbs you want when ordering, so you can get 4 10,000k which would be really white, 4 6500K which would be yellowish or 2 10,000K and 2 6500K and run 1 in each bank.