  We develop a theory for the quantum circuit consisting of a superconducting
loop interrupted by four Josephson junctions and pierced by a magnetic flux
(either static or time-dependent). In addition to the similarity with the
typical three-junction flux qubit in the double-well regime, we demonstrate the
difference of the four-junction circuit from its three-junction analogue,
including its advantages over the latter. Moreover, the four-junction circuit
in the single-well regime is also investigated. Our theory provides a tool to
explore the physical properties of this four-junction superconducting circuit.
