Can i get pregnant if i haven't had my period for 4 months, just have had brown discharge instead of blood? had unprotected sex for the past 3 weeks

Yes: You can ovulate (release an egg) even if you are not having a period. The only way to know if you are pregnant is to do a pregnancy test. If the first one is negative, wait a week and do another test even if you have a period. If not already pregnant, you want to avoid pregnancy and it has been less than 5 days since unprotected sex you can try morning after/emergency contraception/plan b pill.