No idea on cloning the seeded mother. My guess is no, but I have no idea.
Anyhow, the other post is right, you can cram as many plants under that as you want, or grow one big one.
My set up, about half the space (2'x3') I use a 400w and it works well. I would think the problem in your space is getting the light to the ends (of the bathtub). Maybe 2 x 200w one over each half would work, but I read again you already have the 400w.
I am growing 6 plants in that space and they are pretty small. 4 in the space you are talking about would get bigger.
Easy answer is Yes 400w will work for 4 plants in that area. You could use more light, but it all depends on what you are going for. I plan on getting at least a half pound from my 400w and if it was all for me, it would be too much. So if this is for you, go for it. If you are serious about getting the absolute biggest plants and yields, then get another light to cover the area better.
However make sure you can cool things appropriately or else all the light will be a waste, (because you will have to keep it so far above the lights it will be like having a 200w).
I hope that helps a little.
And yeah, I'm doing a scrog my first grow. It is a little complicated and definitely time consuming.