I agree. If lamps are on more than one switch, then you do not have to turn all on at once. And many of us do not - we do dawn/dusk lighting.
I have owned one of those Aquaticlife fixtures in the past, and the lights are split up between ballasts. I liked the fixture; it was much better built than say a Current or Catalina fixture, but not as well as a Aquctinics or ATI.
I would get a 6 bulb fixture if I were you. Like you said, run 4 bulbs, but it will be more flexible for future applications. Buy once is my motto.