We study extended oscillatory systems that respond to uniform periodic forcing at one quarter of the forcing frequency. We find a new type of front instability where a stationary front shifting the oscillation phase by π decomposes into a pair of traveling fronts each shifting the phase by π/2. The instability designates a transition from standing two-phase patterns, involving alternating domains with a phase shift of π, to traveling four-phase patterns. A generalization of the instability to higher resonances is conjectured.