Hood/liner entrance aperture must exceed the size of the cooking area (locus of all pots and pans surfaces on hobs) by an amount roughly equal to the expansion angles (which vary) of the cooking plumes. Generally, we here recommend (at 30 to 36 inches height) a width of 3 inches each side larger than the cooktop. That would be 36 inches for your 30-inch induction unit. 42-inches wide would not be too wide for your 4 ft gap due to further spreading, but a 36-inch wide aperture might be adequate.

The front-to-back depth should also overlap the front of the cooking area by the same amount, and given the sizes of induction cooktops, tiny depths such as shown in the images above are not adequate. 22 inches is good. 19 might be OK. In any case, use baffles, not slots and other arch deviations from commercial practice. Review the Wolf Design Guide (downloadable) for baffled liner examples.

Flow rate should be preferably 90 CFM per square foot of entry aperture. In some cases of only moderate hot cooking on induction, 60 CFM/sq.ft. may do.

Rated flow rate of the blower needs to be about 1.5 times this computed value to account for the blower's fan curve (loss of flow rate with pressure drop across the blower).

Duct size should keep maximum flow velocity in the 1000 to 2000 ft/min range.

Make-up air (MUA) is needed, search topics here and review.

Between those two choices, I would get the 36” with the baffles.

Although, with the baffled part being less then 36” and it is being installed higher then normal, I would definitely go up to the next size.

I know you don’t cook much, but IIRC, weren’t you only going to be living there for a few years and then selling? If so, maybe the new owners would cook, so they would appreciate adequate venting? Sorry if I got that wrong. My memory is terrible! 😬

I've also heard to go wider than the cooktop but it's interesting that with all Thermador cooktops the recommended hoods are several inches shorter... 30" cooktop has two 26" wide vents. I'm not selling in the forseeable future. I'd probably rent it out like the rest of the building if I were to want to move.

The goal is capture and containment of cooking plumes. The plumes spread as they rise. The only way a smaller than cooking area sized vent could work is if it had an excessively high flow rate such that each expanding plume was diverted by entrainment into the hood air flow. 'Excessively' means factors greater than 2, and possibly 10, over what is recommended due to the drop in hood air velocity with distance from the aperture. If the effective hood is actually the structure around the cooking area, e.g., a cove, then full capture is possible if the flow rate is appropriate for the cove's entry area.

Unusual configurations may possibly work, but unless schlieren photography, or computational fluid dynamics results are provided, it is best for residential (height limited) purposes to approximate commercial configurations as they have long been optimized for efficient capture and containment.