i am going to have b'day cake for my son's 4th birthday party. how many candles shall i put for blowing? is there anything like 1 extra candle for luck?

Never heard of this superstition/tradition though. Normally I will just put the actual age unless I celebrate the Chinese Birthday which is rare. I believe all these depends on the individual parents and also if there are old folk around, they may give you different advises as well. Most importantly, the kid must be healthy.