iPhone4 charging voltage?

A good quality charger has enough protection against voltage fluctuations and over current situations. Are you using the original charger?

Apple highly recommends you don't use any chargers that are not apple supported or certified. The voltage on those is much less than the apple chargers are. Apple products require high voltage certified chargers otherwise you will damage the charging station in your phone. I say this because I had an ipod touch and it ruined the charging in it when I used a non apple charger. I have an iphone now and my sister does too and her charging port was ruined by using cheap chargers.