Electric fly zapper - how is this safe?
I don't know if this file zapper meets any standard, or how it does it. But it's extremely unlikely that the 4.7nF cap is 4.7nF at 2.5KV, particularly if 4.7nF is what you measured with your multimeter. 

If you told me that the effective capacitance as 2.8nF I wouldn't be surprised. If you suggest that the working capacitance is 1n5F, I'd just say that I don't have the specifications for that capacitor.