What if this is her fifth life and on her first life she was with the demon king but when ...

I kind of like this theory somehow-

This is more likely her 7th life (considering she's the childhood sweetheart of hero) since she remembers 4th-6th hero as her past lovers (p. 29 ch. 5)

That could be, but in the raw, it just 4 of them so (I've explained it in my comment) but there are possibilities

Well I read somewhere (spoiler) that she doesn't remember her other past lives...