So the dragon will be a loong time to catch ey?
Well, if we compare with the LN & Manga,
- Roxanne, debuted in LN Ch.3 (Vol.1) -> Manga Ch.3 (Vol.1 Ch.3-1)
- Sherry, debuted in LN Ch.16 (Vol.4) -> Manga Ch.38 (Vol.6 Ch.16-1)
- Miria, debuted in LN Ch.28 (Vol.6) -> Manga Ch.101 (Ch.28-4)
- Vesta, debuted in LN Ch.40 (Vol.8)
- Rutina, debuted in LN Ch.55 (Vol.11)
Based on this, it took 13 chapters in the LN & 35 chapters in the Manga for Sherry to debut.
And in case of Miria, she took 12 chapters in the LN, and 63 chapters in the Manga.
The 4th girl takes 12 chapters, while the 5th girl takes 15 chapters in the LN.
So, based on the Manga trend, it could take between 30-60 chapters for the 4th one to appear, unless they rush the plot in the Manga.