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?

I haven't heard of that tradition as well. I have a friend who used individual slices of cake to form the numerical "four". She didn't do it because of any cultural or traditional beliefs but was just trying to be creative. She only used one candle for that cake as a gesture (but come to think of it, her boy had more than one cake since there were many celebrations with different groups. For other cakes, she did use four candles I think. ;p).