http://drupal.org/project/uc_multipurchase

This module allows you to offer "buy X get Y free" discounts in an Ubercart 1.x store on Drupal 5.

The module is designed to allow you to discount the lowest priced product from a given taxonomy term. It also allows you set the minimum number of products to trigger the discount and the discount limit.

For example, if you had a taxonomy term called 'Special offers' with 5 products within it, the cheapest being £5 and set the minimum order limit to 4 and the discount limit to 1 this would give a £5 discount for every 4 products in the cart. So if you bought 8 products you would get £10 off.

This module could allow you to offer 4 products for the price of 3 or buy 4 different products and get the cheapest free.