@Simon364
If the booking was for 4 weeks, it assume it was for 28 nights ?
Then it is a long term stay with "long term stay policy" attached.
Maybe also a monthly discount was applied ?
It also has then a significant lower Airbnb Guest service fee then a short stay.
Shortening (or extending) such a reservation allways is miscalculated due to bugs in the "change" software. If you want to change it, create the change yourself and put in manually the right price.
Please note guest would not have any refund due to the "long term stay cancelation policy", so shortening it to 1 week is big favor you are providing and should include some compensation in the price for you loosing money.