If I understand this correctly, there are eight blocks, and all of them will be used each time - it's just a question of what order they get used in.
So there are 8 blocks you could pull the first time out.
There are 7 blocks left that you could pull the second time out.
Then, there are 6 blocks left that you could pull the third time out.
There are 5 blocks left that you could pull the fourth time out.
The idea is that the number of combinations should be 8*7*6*5*4*3*2*1 = 40,320, based on the above.
- Steve J