Periods can be often irregular, 4 days early is not much but still though its better to get it checked out. If it like that every month then either there is something wrong or it is your own regular pattern your body clock abides too.
I say talk to your GP or doctor and perhaps they will give you some tablets to make your periods more in sync, but it its not posing any major problem (and a health professional has confirmed this) then do not worry or take any medication for it because medication given to affect your menstrual cycle can affect fertility, and it can be in some circumstances unpleasant. Good Luck.