To find the number of stickers, you need to find the LCM of the number of bags.
Number of bags are either 4 or 6.
4 = 2*2 = 2²
6 = 2*3
So LCM = 2² * 3 = 12
So she could to buy 12 stickers. So if she needs 4 bags, she will put 3 stickers in each of them and if she needs 6 bags, she will put 2 stickers in each of them and there would be no stickers left.