Buy 3 Products get 30% off the 4th?

I'm needing to create a "deal" that I can apply to a specific product saying if you buy three of that product you will get 30% off the 4th product, only off the 4th product not the whole order; is this possible with the coupon or discount?

Thanks!

You can't do that with discounts or coupons.
You could create two prices in your product.
One for the normal price, one for the price with a minimum quantity of 4.
If the person buys 4 items or more, the unit price used will be the second one instead of the first one.