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 this superstition. Maybe it's specific to your culture. But if 4 separate candles seem unlucky to you, you can get one of those candles shaped in a specific number. That way, it'll be one singular candle but accurate to your child's age. You can get those candles at the bakery or a party supply store.