  In rhombohedral-stacked few-layer graphene, the very flat energy bands near
the charge neutrality point are unstable to electronic interactions, giving
rise to states with spontaneous broken symmetries. Using transport measurements
on suspended rhombohedral-stacked tetralayer graphene, we observe an insulating
ground state with a large interaction-induced gap up to 80 meV. This gapped
state can be enhanced by a perpendicular magnetic field, and suppressed by an
interlayer potential, carrier density, or a critical temperature of ~ 40 K.
