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i can see your issue here. but wouldn’t you want to aim for a somewhat constant temperature during fermentation? my understanding is that yeast will produce different aroma profiles with different temperatures. so depending on the style you are aiming for on would choose a different temperature. https://www.homebrewersassociation.org/how-to-brew/understanding-fermentation-temperature-control/
in my mind 4°c is sufficiently stable and should be stable enough to not have me worry about volume contraction influencing airlock activity. wikipedia says volume contraction should be minimal at these temps https://en.wikipedia.org/wiki/Thermal_expansion I’d say two days at your average temperature without airlock activity is sufficient to call fermentation done. cheers