Wall Oven and Microwave Vent to Attic
I am in the appliance repair business. I have never seen a Whirlpool oven and microwave that are vented. The only brand of wall oven that I ever saw vented was an older Thermador.

If it is coming from your oven/microwave combo, it would be vented out the top and you should be able to see that in a false cabinet above the unit.

They used to vent the old Wedgewood stoves with a 4 inch vent. I'm wondering if it is an aol vent left over from the original kitchen.   These old vents were always run up through the studs, the vents from the wall ovens that I have experience with were not run up between the wall studs. 

Crawl back up in the attic and take a look at where this vent is coming from. If it is coming from between the studs, it is most likely an abandoned vent.