Welcome to HealthcareMagic forum.
It is good to know that you started regular birth contraceptive
pill as they are better and efficient contraceptive agents. However it takes atleast a month of regular usage to become as efficient as it needs to be.
So it wouldn't protect you against the condom split event and supplementing with additional morning after pill will be advisable.
That being said, presuming your periods are regular and you started yasmin
on the first day of period, the chances of pregnancy
following unprotected sexual intercourse on the 4th day is minimal.
So do not worry. You may take the morning after pill for additional prevention and be relaxed.
Hope this helps.