The transition from Rank Four to Rank Six in the Serpent Dragon Realm constitutes quantitative change, where the majority of cultivators rely on time to make this transition.
The elapsed time largely depends on the innate talent of the cultivator. There's a significant disparity amongst individuals. Even if they reside in the same realm, their absorption rate of spiritual energy and its conversion into Zhenyuan (True Yuan) is different due to unique body constitution. Hence, there's a remarkable gap between an ordinary person and a genius.
Therefore, even though the transition from Rank Four to Rank Six in the Serpent Dragon Realm is relatively easier compared to other significant qualitative changes, not everyone can pass through this phase.