  When a Josephson junction is exposed to radio frequency radiation it
undergoes the inverse AC Josephson effect - the phase of the junction locks to
the drive frequency. As a result, the I-V curves of the junction acquire
"Shapiro steps" of quantized voltage. If the junction has three or more
superconducting terminals, coupling between different pairs of contacts must be
taken into account and the state of the junction evolves in a phase space of
higher dimensionality. Here, we study the multi-terminal inverse AC Josephson
effect in a graphene sample with four superconducting terminals. We observe
correlated switching events caused by the interplay of the connected junctions
on the device. Additionally, we find a competition between trivial voltage
steps, which are created by the device's resistor network, and nonlinear
integer and fractional steps, which are created by the device's Josephson
network. We successfully simulate the observed behaviors using a modified
3-dimensional RCSJ model.
