wow at that size it would be better in a greenhouse or in a room with skylights because that will cost a hell of a lot not only to light but replacement bulbs and running the greenhouse would pay for its self .
That will be easy to light. What types of plants are you wanting? If you just want basic low light plants, and floating plants, then I'd go with 4 HO T5's. If you want higher lighting, then go with 6, 8, etc.
3x400w MH, i don't think there'd be enough coverage to plant the entire pond unless you had them several feet above the surface to spread out(they'd have to each cover a 5'x5' area)
The lights are going to be 4-5 feet above the water. If I do t5's I do not need co2? when you say 4 t5's do you mean 4 fixtures with 2 or four bulbs in each? I would not think that it is only 4 bulbs total.