I've always calculated my cycle the day i finish my period, but it always comes 4 weeks after that last date so do i have a 35 day average cycle?

Yes: If you are bleeding for 7 days and you count the day after you stop bleeding as day 1, then add 28 days (4 weeks), then your cycle would be 35 days. It is not that unusual to have longer than average cycle in your teens or twenties. Typically, however, we start calculating the cycle as day 1 being the first day of bleeding so when someone asks you your last menstrual period you give them day 1.