We study the semi-classical dynamics of four dimensional gauge theories with adjoint fermions compactified on a circle with periodic boundary conditions for the fermions. There are monopole-instantons (with fermion zero modes) and two types of monopole--anti-monopole molecules, called bions. The plasma of the type-one bions--- which carry net magnetic charge---induces a mass gap for gauge fluctuations. The type-two bion is both topologically and magnetically neutral, and its understanding requires a generalization of multi-instanton techniques in quantum mechanics to compactified field theory. We conjecture that it is the semi-classical realization of four-dimensional IR renormalons.