@Michelle2257 You can't really. You could have a max stay of 4 nights and a 25% discount after 3 nights which I think gives 4 nights for the price of 3 but this would restrict your bookings significantly.
Alternatively you could fix Friday and Monday as check in days and Monday/Thursday/Friday as check out days. Then you price the weekday nights 25% (?) cheaper than the weekends which might do sort of what you want (but I am guessing)
Welcome to the Community Center @Michelle2257, I was wondering whether you had a chance to read the comment left behind by Mike? It'd be great if you let us know what you decided to do in the end.