You are only guaranteed a place in a compulsory course if you registered for it during the official registration period.
If you didn't register for courses in period 4, then you can register during the late course registration that runs from 26 January - 5 February. It takes place at the same time as the registration for periods 5 and 6.
Course registration is subject to availability. There is always a spot in mandatory courses of the curriculum, sometimes via a waiting list.