I have already expressed my opinion, so I probably don't need to repeat myself – and there's no need to argue.
Just a couple of points.
Firstly, regarding the third installment. Naturally, I played enough to notice the diversity not only of creatures in the castles but also of the castles themselves – which, in my opinion, is excessive (I used to be very critical about this, considering it a drawback; now I've become more tolerant).
Secondly, the second part of the series was probably not as popular as the third. Try to understand my point – that's precisely why the third installment achieved such success (the second hadn't yet managed to "get under the skin" of the less numerous fans of the series), while being less different from its predecessor than the fourth is from its own.