  For different phases to coexist in equilibrium at constant temperature $T$
and pressure $P$, the condition of equal chemical potential $\mu$ must be
satisfied. This condition dictates that, for a single-component system, the
maximum number of phases that can coexist is three. Historically this is known
as the Gibbs phase rule, and is one of the oldest and venerable rules of
thermodynamics. Here we makes use of the fact that, by varying model
parameters, the Gibbs phase rule can be generalized so that four phases can
coexist even in single-component systems. To systematically search for the
quadruple point, we use a monoatomic system interacting with a Stillinger-Weber
potential with variable tetrahedrality. Our study indicates that the quadruple
point provides novel flexibility in controlling multiple equilibrium phases and
may be realized in systems with tunable interactions, which are nowadays
feasible in several soft matter systems (e.g., patchy colloids).
