Improving the Security of Four-Digit PINs on Cell Phones
The author of this article notices that it’s often easy to guess a cell phone PIN because of smudge marks on the screen. Those smudge marks indicate the four PIN digits, so an attacker knows that the PIN is one of 24 possible permutations of those digits.
Then he points out that if your PIN has only three different digits — 1231, for example — the PIN can be one of 36 different possibilities.
So it’s more security, although not much more secure.