Can I use a 50v capacitor instead of a 25v?
Yes. Just don't go below 25v. The 4.7uF is an "exact" value, whereas the 25v is a maximum rating. Probably yes: Ideally you should replace the capacitor with one of the same nominal capacitance and an equal or greater maximum voltage rating.
Can I use a higher value capacitor?
You can almost always replace a capacitor with one of a higher voltage. This is the limiting factor of a capacitor due to dielectric breakdown voltages that the manufacturer chose. Varying capacitance gets a little trickier.
Can you replace a start capacitor with a higher UF?
An electric motor start capacitors can be replaced with a micro-farad or UF equal to or up to 20% higher UF than the original capacitor serving the motor.]]>