Find the missing letter cluster in the given series:
BYHN, CWKM, EUNL, ______, LQTJ

Let's break down the shifting logic for each position across the clusters:
1. First letter: B (+1) $\to$ C (+2) $\to$ E (+3) $\to$ H (+4) $\to$ L
2. Second letter: Y (-2) $\to$ W (-2) $\to$ U (-2) $\to$ S (-2) $\to$ Q
3. Third letter: H (+3) $\to$ K (+3) $\to$ N (+3) $\to$ Q (+3) $\to$ T
4. Fourth letter: N (-1) $\to$ M (-1) $\to$ L (-1) $\to$ K (-1) $\to$ J

Combining these positions gives the missing cluster: HSQK.