The easiest way to visualize and answer this is to list all possible combinations or arrangements on the 4 sheets of paper.
Take note that the arrangement is as following:
# of stickers on sheet 1 - # of stickers on sheet 2 - # of stickers on sheet 3 - # of stickers on sheet 4
8 – 0 – 0 – 0
7 – 1 – 0 – 0
6 – 2 – 0 – 0
6 – 1 – 1 – 0
5 – 3 – 0 – 0
5 – 2 – 1 – 0
5 – 1 – 1 – 1
4 – 4 – 0 – 0
4 – 3 – 1 – 0
4 – 2 – 2 – 0
4 – 2 – 1 – 1
3 – 3 – 2 – 0
3 – 3 – 1 – 1
3 – 2 – 2 – 1
2 – 2 – 2 – 2
Therefore there are a total combinations of 15 arrangements of stickers on sheets of paper.