And it's actually the stage theme for the final flight of steps into the keep... I don't think that counts as a Boss theme. That's like counting Sigma's first form theme in X6 as Sigma's theme, when it's used in the stage right before the boss.
IMO Simon's theme is the real Dracula battle theme.
That's one thing that always sort of bothered me about SCIV. Such great music... But yet... It doesn't have an actual REAL boss theme for Dracula... It uses the intro/castle keep theme and then Simon's theme. What gives? they run out of money at the last minute on music?
SCIV isn't the only game to have done that. In fact, many CV Dracula incarnations have been given different themes for their different forms.
Among the more famous ones, there's Nothing to Lose and Black Night in CV, Dance of Illusions and Proof of Blood in CoTM, A Toccata into Blood Soaked Darkness - Metamorphosis to the Black Abyss of Death in CoD... And CV64 and LoD have a whole slew of them.
So it's far from being an isolated case.
As for SCIV, since Simon's Theme is, well, Simon's theme, it isn't hard to tell that Dracula Battle is Drac's "
REAL" theme. And it plays throughout most of Simon's final battle.