Having more bulbs is a real benefit. There are different methods to approaching a planted tank (e.g. low tech, high tech) and so depending on which method you'd choose, the amount of lighting would be different. So, if you start off with low tech, for example, and don't need so much lighting, then you could use only two of the bulbs on the four bulb fixture. Then, if you decide at some point to go a bit more higher tech, you could make use of the additional two bulbs, either all of the time that the lights are on or having two bulbs on one timer and the others on another timer. This would enable you...so long as you dose CO2 and apply fertilization properly, to keep a wider variety of aquatic plants, including red and pinker plants for a great variety.
I'd suggest that you get the four bulb fixture and when you're choosing a particular fixture, look for one that has individual reflectors for each of the bulbs.
Another alternative would be to have two fixtures with two bulbs each and place them a bit away from each other on the top of the tank so that you get a broader coverage, but that's up to you and not critical if you get the fixture with the individual reflectors.