This is how you set it up properly...
Unfortunately, if you are using AXIAL in duct fans, I am afraid they will not do the job...
AXIAL fans loose too much cfm too easily...
You need CENTRIFUGAL fans, which can hold cfm under pressure much better than the AXIAL fans...
I say you use a 4" centrifugal fan rated around 170cfm, but remember, it will hold cfm much better, the duct isn't smooth, there are bends...
Better to save up and do it right the first time than waste time and money to get frustrated with less than ideal yields...
My 2 cents...
go with what you know...