I know Buffy says that she's only had 4 lovers but you know what those Summer's girls are like at denial
Buffy has never been in denial about having sex with someone she did have sex with, or about something that had happened to her. She might avoid dealing with her feelings, but she's not delusional about the things that she actually did or that actually happened to her.
And if the story was that she had sex with Dracula, we would know. I don't believe that there are things that happen off-screen that the writers are keeping from us so that they would reveal it in season 15. Not to mention that this certainly didn't happen in season 8 when the character of Dracula was brought back, and the relationship that was focused on was Dracula/Xander, with no romantic or erotic vibes between him and Buffy.