When capacitors are in parallel, the lowest voltage rating applies. Just curious, why would you put 120 uF in parallel with 4700 uF? The value of the smaller cap is less than the tolerance of the larger one.
just the clarify, the arrangement would be rated to 6.3V, anything above this and you would damage the larger capacitor.
I think what he was trying to achive was a large, high voltage capacitor, sorry mate it doesn't work that easy. If you want a high voltage high capacitance capacitor you going to have the shell out a little for one, not just bodge two random ones together.