There are no 5th-level creatures in the 4th game, and that's why the Citadel is considered a 4th-level structure.
P.S.: Those who have played both games will understand me.)))
Added 2 minutes later
Of course, bonus monsters like the Sorceress, Gargantua, Dragon King, etc., are significantly stronger than the traditional 4th-level monsters that can be recruited in a castle... Even though there are no 5th-level creatures, it should be acknowledged that they form their own, special class.