I am feeling worried because my period usually last 6-7 days (although it slows down a lot by day 4) and this time, it only lasted 4 days. what can be the cause and should i be worried?

If pregnancy is possible, a short bleeding episode may not be a "period", and may represent a pregnancy outside the uterus (ectopic). See your gyn provider for an exam, who may order a serum HCG level to be certain about pregnancy. Amounts of bleeding do vary with some cycles providing low hormone support of endometrial cell growth, hence less bleeding.