Ok I misread your schematic and thought it showed a 4.7uF capacitor not 4.7nF, so yes in that case a quarter watt would be fine.
It went through last winter which was exceptionally cold in the UK so the heating was running quite a bit.
Can't help thinking a 4.7 uF high voltage capacitor would be quite a size. The .01uF was the highest value my supplier stocked- about 20mm dia 
Here it's really cold right now, I don't want to think about when it's really winter...
Waking up when it's cold it's terrible!