Why would I "totally understand this" if I were Korean? If you're so sure it's okay, why ask about it on Yahoo Answers?
I'm not Korean. I just lived there for five years and went to their schools, and worked in their economy.
Four years is at the upper limit of acceptable age difference in Korea. Traditionally it was considered optimal.
That's what traditional Korean culture says, but frankly, an 18.5-year-old guy (probably graduated from high school and worked in some capacity) and a 15-year-old girl (still a complete dependent of her parents) have nothing in common. And it's illegal if this is in the US. Let's not forget age of consent laws. Of course, I guess that doesn't matter much to your sister since it's not her who would be thrown in handcuffs and gang-raped by big, burly inmates in a maximum security prison for the next 20 years.
I'm 22 right now. I work full-time. I've been living overseas continuously without my parents' support since I was 19. I've graduated college. What the hell do I have in common with an 18-year-old high school girl, really?
And when the people are younger, a four-year age difference is even worse than that.
Unless the guy is exceptionally immature, or the girl is exceptionally mature (oh, and they all think they are, but are they?) I can't see how this could work.
Of course, I'm sure your sister will do it anyway. Nothing makes a girl more interested in a guy than knowing he's dangerous.