  We investigate the direction-dependent switching current in a flux-tunable
four-terminal Josephson junction defined in an InAs/Al two-dimensional
heterostructure. The device exhibits the Josephson diode effect, with switching
currents that depend on the sign of the bias current. The superconducting diode
efficiency, reaching a maximum of $|\eta|\approx34\%$, is widely tunable - both
in amplitude and sign - as a function of magnetic fluxes and gate voltages. Our
observations are supported by a circuit model of three parallel Josephson
junctions with non-sinusoidal current-phase relation. With respect to
conventional Josephson interferometers, phase-tunable multiterminal Josephson
junctions enable large diode efficiencies in structurally symmetric devices,
where local magnetic fluxes break both time-reversal and spatial-inversion
symmetries. Our work establishes a pathway to develop Josephson diodes with
wide-range tunability and that do not rely on exotic materials or externally
applied magnetic fields.
