I haven't got my period for 4 months

Is there a chance of pregnancy? If so, have you tried a pregnancy test? Are your periods usually very regular? Have you see your doctor? How old are you?

There are a number of possible reasons, obviously pregnancy being high on the list if you are sexually active. If you are young, it may be due to that. Teens will sometimes find they have irregular cycles from time to time.
Some types of birth control can cause periods to be irregular or stop altogether for some people.

So it is time to go to the doctor, if you are pregnant, you need to know and if not, you need to find the problem