I recently finished it too and I couldn't agree more! It's a heck of a series and this 4th season was very bittersweet for me because, while I think it was a great wrap-up for the series, I really wish it would go on.
I am a hardcore fan of the Castlevania games and I wasn't disappointed at all. On the contrary, it hit me with a lot of nostalgia and good memories!
I really hope that if we do get a spin-off, it's based on the Castlevania: Symphony of the Night game, which is, in my humble opinion, the best game of the series and one of the best games ever made! The game features Alucard and one of Trevor Belmont's descendants among its main character so they could make it work!
Good to see that fans of the game were not disappointed!