The "fourth die" is a completely optional add-on. The DM's Advice Dice will work fine without it, but for any backers interested in obtaining it, there are pledge levels that include it for a slightly higher price. (Note that all Limited Collector's Editions have one included.)
Every reward (either one set of DM's Advice Dice, or the "share with friends 3-pack") is available with or without the optional 4th die.
Full details are available in Update #6: https://www.kickstarter.com/projects/2cnk/dungeon-masters-assist-dice/posts/2792423