I'm 15 and have had my period for about 4 years, is it normal that its still irregular??

Yes.

It's not that unusual. Sometimes it can indicate a problem but not always. It's worth mentioning it to your doctor next time you go, or make an appointment if you're worried about it.

It is very common among girls who are thin or are athletes to have irregular periods, or for them to even stop completely. I didn't have periods all season when I was a high school swimmer - I didn't have enough body fat, per my doctor. When season ended and I gained a little weight in fat rather than muscle, it started up again, then next season it would stop again. In college, none for four years since I was a competitive swimmer year round. So, if you're an athlete, or are just naturally thin, that could be the reason and you do not need to worry about it.