Lucky Sum is an additional game you can play with Win 4 and Numbers. It costs an extra $1 and gives you the chance to win a prize if the sum of your chosen numbers matches the sum of the winning numbers. For example, if the winning line 0356 is drawn, you will win a prize if you pick 1607 because both combinations add up to 14.
The amount you win depends on which sum you match. For example, in Win 4 the only combination of numbers that adds up to 36 is 9999, so the prize in this category is much higher than the prize for matching a sum of 18, for which there are 670 possible combinations.