What is differences of WV and V for Capacitors
I was told once that it stands for "working voltage"; at least, that's what the photo flash community transports.

So, it's the same: a voltage rating. For guarantees made when using that voltage or below, you'll have to read the datasheet for the individual capacitor, anyways, so your question is (and I mean this positively) kind of pointless: 

Both numbers suggest you can "reliably" use this capacitor at 400 V (you probably guessed that already). But what "reliably" means, exactly (how long, at which temperatures) and what influence charge/discharge currents and so on have on the capacitor can't be answered without a datasheet. The datasheet would also define what "400 WV" means.