Currently the Airbnb platform does not allow for discounts for a four day stay. Discounts can only be set for 7 and 30 day stays. For a seven day stay just set the discount equal to a night free.
The only other way to do what you ask is to write your listing to encourage people to contact you to request the discount, then send them a special offer that would be priced at the three night charge for four nights. It's not automated, but it keeps you in the loop and makes it personal for your guests. It also gives you the opportunity to screen guests while leaving instant book on too.