Flow through a 4-inch vent at appropriate speeds (1000 to 2000 ft/min) for minimum grease condensation due to cold or to impingement will be rather limited for any cooktop ventilation purpose. A kitchen reno should not only include a hood with the appropriate characteristics to achieve cooking plume capture and containment, but also a suitable duct size and blower rating. Deliberate supplemental make-up air may also be required.

Best to start with your cooktop size and go from there.

"Best to start with your cooktop size and go from there."

This.

Using a reducer as recommended above can create a fire danger if you do much cooking that creates greasy effluent. Depending on the blower it can also significantly shorten the life of the blower.

It really depends on your range, what you cook, how often you cook, what environment you're in and how often you keep a lot of windows open for fresh air.

Do you have the ability to enlarge the duct? How long is it and what is its path of travel?

Im not sure! i have a 30” induction range. i don’ cook a lot of greasy foods, just bacon occasionally,