Splitting a 12" trunk flexible duct...please help!!

[Linclink]

Ok...I live in a mobilhome and I'm tired of possums screwing up my ac ducts. So I'm in the process of doing a ductwork and make it come from the top of the house

My trunk duct that comes from the package unit is a 12" diameter duck, and doing some calculations ( in terms of area) I realize that I would have to use around 9 ducts of 4" diameter to keep up with the same area of a 12" duct.

As the pic below shows, I was thinking about 4 splits of 4" ducts. Now I'm not sure if it will be enough to release all the air coming in.

Any help will be really appreciated!!