They start younger and younger

For the past year or so my son has referred to Katie as his girlfriend. Apparently, they are a known item at the preschool. At first, my son was hesitant. He came home and retold a story that included him telling Katie, "Every since I said you were my girlfriend you follow me everywhere!". He just needed some space.
Since that time, the relationship seems to have solidified. Not long ago, my son was a bit concerned because a princess could only have ONE prince and he was Katie's prince. But, Jacob also insisted on being Katie's prince. As Katie seems to be very popular with girls and boys, my son has also had to put his foot down with Taylor who insists that SHE is the person who should sit next to Katie every day.
Just last night at dinner, my son kept us entertained with his story about how he and Katie kiss at school. I asked if they were allowed to do that, to which he answered [all smiles], "No, we sneak to kiss".  I further inquired if they kissed on the cheeks or on the mouth. The smile was permanently stuck on his face as he explained that they can kiss each other wherever they want. He repeated this several times to make sure we understood.

they seem so innocent, but when you're not looking...

OK -- he's 4 -- this is harmless right? Still, I am torn between just letting it go and not making a big thing of it or asking the school to keep an eye on them or telling him that he shouldn't be doing that. I've asked some friends who seem all too eager to high five him. That seems wrong. But, what is right?