2-4 episodes sound little. It can work with the first books, but from Goblet of Fire onwards, it would need more episodes. While that increases shooting times, it also offsets fixed costs like sets and wardrobes. Of course, it also means more revenue. And while CGI at that time isn't up to scratch (or it's too expensive. Lord of the Rings CGI holds up perfectly well up to today, but I doubt a 2000s TV show will expend that much), there are also fixed costs in 3d models, as Game of Thrones show. Once you've modeled a creature (ie, dragons), you still have the costs of animating, rendering and composing the creature in the scene. But the cost of modelling and texturing the creature is also offset in as many episodes you include it. So while it's non trivial to add acromantulas to the third task of the Triwizard Tournament, if they were already designed for season 2 (if), then they can repeat at the cost of animating, rendering and composing. The same for dementors, house elves and goblins.