We have a 1 year old GE microwave range hood but it directs/exhaust air
thru a 4 inch round duct to outside the roof.


4" is too small for a duct.Usually it specs out for at least 6" or 7"
round duct. It's in the manual.

Ducting to the outside is a definate plus. When I re-did my kitchen
years ago, I gutted the entire area, and noticed there was a stud in
the way where I was going to run the duct up to the roof. Not having
ordered the cabinets yet, I re-designed the layout so that I moved the
stove over another 3" to avoid the stud, thus having clearance to run
a 3 1/4" X10" duct in between the studs, out through the roof.