duct reducers

Narrowing of a lumen increases the backpressure while increasing the velocity (speed) of the air. It's called the Venturi Effect if you want to know any more than that.

Cfm isn't speed though it's volume. Increasing back pressure means you are slowing throughput so yeah the cfm will be diminished.i don't know by how much or what math you would use to figure it out.

Is there anyway you could split it into 2-4's? That would give you much less restriction. As for how much restriction you are going to cause. A 6" hole has 28.27 sq. in. A 4" hole has only 12.57 sq. in.

If you had 490 cfm at the reducer( which you probably don't due to friction, bends etc.) I am told you would lose about 100 cfm overall. Velocity would increase due to increased static pressure. Increased static pressure will most likely cause your fan to work about 25% harder.
HanginIron hit the nail on the head when he said to use a wye w/ 2 - 4" ducts. That would equal a 6" line.

yeah I was thinking about using 2, 4 inch holes. I just dont want to make it obvious bc I was told by the assosiation i cant add a vent to the roof. I only have a ridge vent along the top of the roof and dont want mold in the attic.

They do make sq to round boots for air registers out of sheet metal. You know ..like they use for ceiling rough ins. I don't know if that would be an option for you but it is something to think about. If not, what the hell..a round 4" will work