I'm pretty sure I'm running 2 blue plus, 2 actinics and 2 purple plus. I MIGHT have an aquablue special in place of one purple plus, but I don't remember. Either way, I recommend buying the 7 bulbs I listed so you can pick your favorite combo. If you're using a 4 bulb, I'd recommend 2 Blue plus, 2 purple plus and an aquablue special. Pick what looks best out of the group.
You'll have to store a spare bulb, but you can use it when you do a bulb change in about a year.
If you have a robust local community, you may also be able to get some people to give you some old bulbs they're throwing out. I wouldn't run them long term, but they'll be just fine to dial in the color combo you like best.
Lastly, don't rule out supplementary LEDs. You don't have to buy high end fixtures in this case, just some blue strips that will add some extra 'pop' in the tank.