If we only reach $15k and you backed for 4 notebooks then all we would be able to print and send you is 4 Carbon notebooks.
Fortunately, Kickstarter is very flexible with allowing backers to change their pledge while the project is still live so you have two options:
1) Pledge for 1 now, then change it to 2 at $25k, 3 at $35k, and 4 at $40k
2) Pledge for all 4 now, then change it down to 1, 2, or 3 before the campaign ends if we don't reach $40k
Don't see the answer to your question? Ask the project creator directly.