Minimum voltage rating for output capacitor for use with LTC3525-5 boost voltage regulator
I simulated your circuit using LTSpice for 3V in and 4.2V in and there was virtually zero overshoot. That means you could use 6.3V capacitors. If your local supplier doesn't have any you can always order offline. You could probably get 10V caps just to feel safe and increase the robustness of the circuit if area and cost aren't super critical to this design.