@Jocelyn653
You can't offer a "free night" on Airbnb...I believe the minimum is $10? Not sure? You can use a Rule set based on min stay with XX% Discount, but not free. You could try playing around with the maximum discount the Rule Set allows?
What you can do is advertise on the listing in the description:
"Ask us about staying 3 nights and get the 4th free"
Then if the guest messages you about the free night, send a special offer for 4 nights to include only the 3night rate + cleaning fee. Of course this only works for request to book, not instant book. If using instant Book, then have the guest cancel the booking and send an inquiry instead, then send a Special Offer.
Some Hosts also message guests and advise if they would like to extend their stay an additional night either before or after the stay, they will offer a XX% discount.