D4
A pyramidal die that has the second-sharpest points of any die (only the d8 is sharper), and, appropriately enough, has four sides. Used by Wizards, small weapons, and low-caliber firearms in d20 modern. At least it gets more love than the d12, which probably falls asleep every night in a pool of tears and melted butter.
There are two ways to print the numbers on a D4. On one, the numbers are arranged on the corners of each face, and so the number at the top (it will always be three of the same number) is what you actually rolled. On the other kind, the numbers are arranged in the middle of each side of the face, and so the number on the bottom (again, it will always be three of the same number) tells you what you rolled. Oldfags will insist the "numbers on the bottom" d4 is the one true way, despite the fact that they need the full power of their coke-bottle glasses to see the numbers. Don't argue with 'em, just keep using the d4s that you and everyone at the table can read.
Fucking Caltrops
Whilst the d4 isn't as sharp as a d8, it has one major bit of natural defense - no matter what way it lands, it will have a point face-up. Because it's the smallest die, care needs to be used - if one escapes its dice-box and into the wild, it will wait, with its natural weapon ready, for the exact moment someone walks into its vicinity barefoot to strike, whereupon it will inflict some surprisingly-vicious puncture wounds.
Somewhat related, in D&D Caltrops inflict 1d4 damage. Coincidence?
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A d4 in the wild, natural weapon readied. As you can see by the notch on the left edge, this one has already claimed a victim.
