  For a prime $p$, we determine a Sylow $p$-subgroup $D$ of a finite group $G$
such that the principal $p$-block $B$ of $G$ has four irreducible ordinary
characters. It has been determined already for the cases where the number is up
to three by work by R. Brauer, J. Brandt, and V.A. Belonogov thirty years ago.
Our proof relies on the classification of finite simple groups.
