Replace the vent through the wall or buy a cheap hood that works with a 4” duct. Buying a hood that requires a 6” duct and then forcing it through 4” is a waste of money.

Thank you. Another issue that I can’t even find chimney style wall hood that uses 4” duct. Any suggestions?

Well, technically, the more a blower's air flow is restricted, the less work it does and it is unlikely to become "run down" as a result, unless it over-speeds due to a near total blockage.

Blowers all have fan curves, as illustrated below, that represent their air flow rate as a function of pressure loss. Pressure loss is the pressure across the blower (zero hanging in free air). This pressure includes both the back pressure on the exhaust side and the decreased pressure, if any, on the intake side due to lack of make-up air. A properly designed blower can operate over most of its fan curve space. Using a higher rated CFM blower (always specified for free air conditions -- see "Maximum free delivery" arrow in figure) to overcome pressure loss is an option. You will, of course, have a cost, and this will be mainly noticeable as increased blade tip turbulence noise and duct turbulence noise, although the latter will perhaps be modest in a very short duct.

Try for a tapered reducer that avoids abrupt diameter transitions. You may have to have one fabricated at a HVAC distributor that provides duct fabrication services.

(Click to enlarge)

Thank you kaseki. If I use reducer should I buy range hood with 450 cfm instead if 700+ cfm to reduce noise? Also, should I maybe put reducer at the end of duct exit outside?

"Another issue that I can’t even find chimney style wall hood that uses 4” duct."

None of the wall-mount range hoods at build.com use a 4" duct. Owners like these 350cfm, 6" duct hoods:

• GE JVW5361SJSS 350 CFM 36 Inch Wide Wall Mounted Range Hood
• Whirlpool WVW53UC6LS 80 - 350 CFM 36 Inch Wide Wall Mounted Range Hood

4” duct is a dryer duct, not a range hood duct. It must be changed out. It should have passed inspection and exist in the first place.

Thank you Zumi. House is 20 years old and i had microwave hood there before

The rated CFM required depends on the hood, first and foremost. What are its dimensions? Is it conventional or some entertaining diversion from standard practice? Does it overlap the cooktop as is desirable, or just part of the cooktop?

It will be 30” hood with 19” deep that will cover all flame

• ZLINE KB-30 120 - 400 CFM 30 Inch Wide, 18" deep, 6" duct, Wall Mounted Range Hood With Stainless Steel Baffle Filters
• Master Flow R6X4 6 in. to 4 in. Round Reducer

"Q: Can this be used with kitchen range hood. Currently I have a 4" pipe that vents straight up through roof without bends and is about 6 feet long. The new range hood has opening of 6".

"A: You should not decrease the size of the range hood exhaust." -- Master Flow

A nominal 30 x 19 has an area of 4 sq. ft. A flow velocity of 60 ft/min (light cooking) to 90 ft/min (hot oil cooking) is desirable. 4 x 60 =240 CFM; 4 x 90 = 360 CFM. To get rated from actual a rough factor of 1.5X may be sufficient, leading to rated blowers of 360 and 540 CFM, respectively. I think the baffle pressure loss will dominate over 'scrunching' a 6-inch exhaust into 10 inches of 4-inch duct, but the relevant fan curves are needed to evaluate behavior.

An on-line calculator produced a pressure loss of 0.075 inches, w.g., for 1 ft of 4-inch duct at 360 CFM (0.035 inches, w.g. for 240 CFM). At 360 CFM, the air velocity in the duct would be twice the recommended maximum for grease particle impingement, but this may be something easily addressed by occasional clean-out efforts if necessary.

Below is an example fan curve for the Broan/NuTone 331H, 630 CFM rated blower. One might infer from the curve that expecting 240 or 360 CFM would be feasible. as there is plenty of pressure loss "available" beyond that of the 4-inch duct loss to allocate to the baffles, hood transitions, and make-up air losses (assuming some means of make-up air).

This may not apply to whatever blowers might be options for internal use in the hood.

I personally would cut the hole in the siding larger if practical.

Cut the hole. Make the duct bigger. It's criminally small.

If you will be using your range then for your and your families health you need proper exhaust. A 30" gas cooktop can produce just as much harmful effluent as a 48" gas cooktop.

See: https://bamasotan.us/range-exhaust-hood-faq/

Thanks everyone for your comments