This paper presents a controller for the DC voltages on the split capacitor topology for a four-wire selective active filter. A simple model for the dynamics of the two capacitor voltages is derived under the assumption of timescale separation between the dynamics of the inverter currents and the capacitor voltages. The controller proposed is based on physical principles and can be easily integrated with existing four-wire active filter controllers based on instantaneous reactive power (p-q) theory. Simulations and experimental results illustrate the benefits of the solution proposed.