[vba616]

Here's a riddle.

If using my birthday reduces the entropy of my PIN, what does it do to its entropy if I happen to have the same birthday as one of the most famous people in the world? Does it matter if I am aware or not? Does it matter what they use for their PIN?

For the sake of argument, I'm thinking month and day, not year.

[rcxdude]

The important thing is that your PIN has zero entropy, regardless of its value. Entropy is a property of distributions, not individual values. You may be thinking of the probability (or information content) your PIN is assigned when looking at the overally distributions of PIN, in which case it probably does matter how popular your birthday is (and whether it also matches common patterns people use for PINs). This does feed into the calculation of entropy for the distribution but then it ceases to tell you anything about your PIN specifically. It also only makes sense when you are looking at it relative to the distribution, so it matters how you specify the PINs you are comparing it to.

The 'information content' of a given outcome is the logarithm of the inverse of its probability (i.e. more unlikely events give you more information), and the entropy of a distribution is the expected value of this information content.

[diffeomorphism]

That is not a riddle just a question how to handle information on the distribution of passwords in the population. So you get the same answer as if it were four alphabetic characters and your choice is "soup".