Because you have just started your period last year, it is normal for your menstrual cycle to be irregular. This means for the next [average] 2 years after you first began your period, you could go a week, month, 3 months, 5 months, or even a year without your period. One time your period may last for 3 days, another for 7, and then for 5 days.
Usually [average] 2 years after you start your period, it will become regular. You will be able to predict when your period will start, how heavy/light it will be, when it will end, and how long it will last. -- Although, *NOT EVERYONE BECOMES REGULAR* -- Some people are irregular their whole life.
If it does not start to become regular within the next year or so, or if it truly concerns you as of right now, speak to your doctor.
Your diet, stress levels and sleep levels may contribute to the regularity of your cycle. Be sure to eat healthy and be active for at least 30 minutes-1 hour per day. Sleep well (at least 8 or 9 hours per night for your age) and be happy! Also remember to drink lots of water.
While it's *very unlikely*, this type of irregularity can be caused by a medical condition. Nurses/doctors (at least where I live) typically say that if you've just recently started your first period, you should not be concerned about not having your period unless you've gone 3 months or more without it. Considering you've gone 4 months without it, it's best you speak with your doctor just to be safe.
When I was 12, I went nearly 6 months without my period. I started at 11, months away from turning 12. -- In the end, I was totally healthy and normal, but it's better to be safe than sorry.
Good luck! :)
And, it's best to talk to your mom or sister about it (considering your sister is older, feel free to talk to whichever you feel more comfortable with. If your sister is around your age or younger, talk to your mom or maybe even an aunt or cousin). Considering you are not sexually active, it's obvious you're not pregnant and you definitely don't have any STDs/STIs (Sexually transmitted diseases / Sexually transmitted infections). It's very likely that you're alright, but in the absolute worst case scenario, the medical condition most likely to be causing a lack of menstruation would be anemia (which I doubt you have, but check with your doctor). Your mother, aunt, female cousins, grandmothers, and even your sister (if she's older) have ALL been in your situation in some way or another. Do not be embarrassed to talk to them about this. It's important to be open about your concerns, especially if they're medical-related or concerns about your body. They could help you a lot in the end - and you won't regret it.