Möbius function on a finite poset (X, $\leq$)

Question

I'm having some difficulties with the following problem:

Give an example of a finite poset $(X, \leq)$ and elements $a,b \in$ X such that $\mu(a,b)=-4$ where $\mu$ is the Möbius function of $(X, \leq)$.

I think I know what a finite poset $(X, \leq)$ typically looks like, but how does this Möbius function come into play? My understanding of Möbius functions is mainly limited to recurrences and matrices, so I don't know how this generally changes things.

Answer 1

A simple example is the following: