Would it use just the 1W for the 8 hours it was turned off each night, or is that 1W per hour etc?
Considering electricity is about 10 cents a KWH...
4w x 365 x $0.10 / 1000w = $0.14
So you'd be saving about $0.14 a year by switching your power supply off in the back every night. Granted, this is all theoretical, and based on a guess, but even if it actually used ten times the amount of power per hour while the computer is off, I'd still say that it's not worth breaking my back over to switch it completely off every night.