My bottom cake is 8" triple layer.
Top layer is a 4" triple layer.
Both are circles.
Does the top layer really need dowels and a cake board since its so small?
Instead of dowels, I would use straws, but yes, I would support the 4 inch just to be safe. I wouldn't want the risk of any mishaps.
Yes - I have the bubble straws out and waiting in case I needed them. Would 3 be enough.. seems like more is superfluous.
I think 3 would do the trick. Good luck!