4 year old kissing boys at school.. how do I approach this? Lol

So my daughter(Jk) started at a new school a couple weeks ago. She now takes a school bus and about the 3rd day in, the first thing she tells me when she gets off the bus is “me and Alec fell in love today, and we kissed on the bus” ..on “the lips” lol

Which I mean, a part of me is actually super happy she’s openly telling me this stuff. But how do I tell her she can’t be doing that stuff so young without making it awkward/making her worried to tell me things in the future?

We told her kissing is only for her family etc and told her no more kissing him.. then today she told us “she forgot we said not to kiss Alec so she kissed him today on the bus and yesterday”..🤦‍♀️

Her dad is not very happy😂😂

I am lost on how to approach this.

I know she doesn’t understand why we don’t want her doing that, which makes it harder to explain.

I worried about everything else when she was starting school, but boys being all over my daughter in JK was not something I thought I would have to deal with lol..(she’s also “inlove” with another boy because he told her she’s the prettiest girl in their class) lol I tell yah..🤦‍♀️ I find a lot of it adorable and hilarious but I’m really trying to find a way to approach it seriously because it is an important subject . Help me!!!