Capacitor matching/replacement - different voltage
These are motor capacitors, not electronic.

4.5 280v/ 5 250v/ 5 250v means the cap box has 3 separate capacitors inside, one that is the main Starting capacitor, likely the 4.5uF 280V one, and the other two are for speed changes. So low speed is both caps in series, middle speed is one of them, high speed is neither. The starting cap is rated at a higher voltage because it will ALWAYS be used on the circuit, regardless of speed, the other two speed caps will always be in series with the motor, so they see less stress. 

The voltage rating is the working voltage for the capacitor to reach 60,000 applied hours. It needs to be AT LEAST the rating specified on the motor nameplate (which you may not be able to see on a ceiling fan) but as a general rule, the higher the voltage rating, the longer it will last. So will a 4.5nF 250V cap work? Yes, but not as long as one rated for 280V. If your line voltage is 240V, you are on the ragged edge of acceptability, so you might find yourself replacing that cap again sooner than later. If you are in North America (you didn't say in your profile) and using 120V for the fan, you may never notice the difference.