Olivia's kissing

Olivia got into some trouble today at school. Apparently she's been kissing and hugging too much at school. Specifically her "boyfriend" named Mason. Last time we had a discussion about Mason, she told me, "I can kiss Mason if I want to, Mommy!" ... I decided to let it lie because as far as I knew, this kissing was hypothetical. Mason has been her "boyfriend" since last school year, and she has been constantly telling us that she is going to marry Mason. Since then, the kissing has become less than (more than?) hypothetical. Ms. Dianna stopped me at school today to inform me that she had to talk to Olivia today about it. When I got to school, she and Mason were sitting together hugging and talking. Then I talked to Mason's mom who has determined from Mason that the kissing has happened on at least one other occasion. I felt like I was discussing a teenager. We had a long conversation with Olivia at the dinner table tonight how kissing at school was against school rules and that our family had a rule too: no kissing outside the family. I feel a little bit better since she agreed to follow our family's and school rules ... but I'm still nervous. How can a four-year-old be so enamored of a boy? And what does this bode for teenage years?