Since you state you're just trying to replace it, look at the highest voltage rating on any marked component on the board, and go with that rating.
The physical size of the capacitor is a clue -- if the substitute cap is the same diameter or larger, it should be fine (actually, the thickness is a better clue, but harder to judge.).
That said, 4.7 nF ceramic caps are inexpensive at any voltage, and a mistake picking a higher-rated cap would cost little more and have no negative consequences -- as long as it fits.