This Y with 6" female and two 4" male branches usually comes with several dust collectors. It attaches directly to the fan housing.

You might decide to connect a 6" metal tube directly to your dust collector and run 6" branches around your shop.

This fitting, when mounted on the ends of your 6" lines, is perfect to drop the diameter down to two 4" branches exactly where you need them... on the end of the line.

I have my own shop set up this way but I used the smaller 5" duct and the #13068 5 x 4 x 4 Y. If you decide to use 6" duct and this 6 x 4 x 4 Y and you want to press a 4" female blast gates onto the 4" branches, use a heat gun to heat up the female side of the gates before pressing them onto the Y. This Y has 4" branches that are a little larger than usual so the gates don't slip on without heating. As mentioned on the #13068 page, I did exactly this and I have the most solid connections you can imagine.