I'm not sure what you mean. 'Tier' isn't a meaningful distinction in this case. They were released at the same time, tuned similarly and Void Reaver/Hydross were the clear next step. As soon as you killed Gruul/Magtheridon you were attuned and you could go do them.Additionally, some bosses just seem really out of place, or at least the facts that are put next to them. Bosses like Hydross and Void Reaver had a long gap because they were apart of an entire different tier while simultaneously being opened with T4,