This tutorial is for those looking to offer multiple products that are the same price for a discount after the purchase of a certain number. This example will show you that if you buy 3 different products for $12 that are in the same category, you can get the 4th free.
In the back of Magento go to Promotions > Shopping Cart Price Rules. Under Rule Information enter the information below. You can change the name and start date accordingly. This example will automatically calculate the discount in the shopping cart without having to place in a coupon code. If you would like to require the code you can select Coupon instead of No Coupon and place in the code of choice. You can also select if you want the user to only be able to use it a certain number of time under Uses Per Customer (0 represents infinity).
Next go to Conditions on the left and enter the information below. However change the Category ID from 59 to your ID, and the number of total products in the cart that will initiate the discount. You can find the Category ID in your Categories (Catalog > Manage Categories) by selecting the Category of choice and looking towards the top of the page for its ID number.
Next continue to Actions on the left. There has been issues with the Buy X get Y for free in the past. So we decided to over a fixed amount discount that is the same price as each of the other items at $12. In your case change it to the amount of your products, but they have to all be the same price in this case so you can get the last one free.
Once complete select Save Rule in the upper right hand corner and you should now be able to purchase 3 different products in the same category that are the same price, and receive the 4th free.