[GriffinRU]

25V rated capacitor can be used for 4S applications, but I do not recommend doing this.
Price and parameters wise there is no saving, but there are benefits running 35V rated capacitor with ESC's capable running at higher voltages. In addition, the higher the voltage rating, the longer the capacitors life, unless you plan to replace them as tires...Not to forget running in hot environments stress this ratings further.

Bottom line rules:
To properly select capacitor voltage ratings user must add 25% to the operational voltage (including high voltage spikes due to the backEMF) and select capacitor with rating higher then calculated value.

4S LiPo -> 4*4.2=16.8 -> 16.8*1.25=21V -> 2-3 volts for spikes 21+2=23V
Based on this calc, capacitor rated for 25V would work fine, but with small safety margin. By the way this ratings usually at 25 Deg C, with Temps going up rated voltage goes down.

5S LiPo -> 5*4.2=21 -> 16.8*1.25=26.25V -> 2-3 volts for spikes 26.25+2=28.25V
25V rated capacitor would fail in this application.