@Hans192 Noise from construction work isn't one of the Travel Issues listed in the Guest Refund Policy, but if the guest was able to persuade an agent that the construction zone created a safety hazard your property, it might be considered grounds for a refund. Wood lice aren't known to be a health risk, but I think there's no disagreement that the experience fell short of what was presented in the listing due to this unfortunate combination of factors beyond your control.
If Airbnb were to refund the guest's service fees, it would be at your expense (see sec 4.2 in the link above), but that usually only happens in extreme cases where Airbnb has to intervene to relocate the guests. It's hard to predict how a dispute would be resolved if the guests escalated their resolution request, but if you decide to make a counter-offer, your calculation (guest's total payment divided by 5, times 4) seems completely fair to me.