I don't think hydro makes it more potent, but definately faster growth and bigger yield. A little more cost upfront though since you need to maintain pH and ppm with more precision than with soil.

HID's IMO are worth it in terms of increased yield over fluros. But fluro's work okay. You can grow 4 plants many different ways. I don't see why you'd need a 400 for 4 plants. It all depends on how you grow them. If you grow them single cola at 1 per sq ft you'd have 4sqfeet, a 250 would more than adaquately cover that area at over 50 watts persqfoot. Yes with a 400 you can veg longer due to the bigger footprint and you'll end up with a bigger yield.

With lighting don't go by plant number, go by space. You want roughly 50 watts per sq foot, so a 400 should cover about 8sq feet, a 250 would cover about 5sq ft, etc.