Solution: Assume that the fixed cost of books is X yuan and the ratio of variable cost to the number of books is Y. 。

Then the equation can be expressed by the formula:

x + 2000y =2000*8

x + 3000y + 5000 = 3000*8

X = 10000 y=3。

When printing 4000 copies, the profit is

4000 * 8-10000-4000 * 3 =10000 (yuan)