Which letter cluster will replace the question mark ($?$) to complete the given series?
HEWC, KIRI, ?, QQHU, TUCA

Let's analyze the pattern for each position of the letter clusters:
1. First Letter:
H (8) $\stackrel{+3}{\to }$ K (11) $\stackrel{+3}{\to }$ N (14) $\stackrel{+3}{\to }$ Q (17) $\stackrel{+3}{\to }$ T (20)
2. Second Letter:
E (5) $\stackrel{+4}{\to }$ I (9) $\stackrel{+4}{\to }$ M (13) $\stackrel{+4}{\to }$ Q (17) $\stackrel{+4}{\to }$ U (21)
3. Third Letter:
W (23) $\stackrel{-5}{\to }$ R (18) $\stackrel{-5}{\to }$ M (13) $\stackrel{-5}{\to }$ H (8) $\stackrel{-5}{\to }$ C (3)
4. Fourth Letter:
C (3) $\stackrel{+6}{\to }$ I (9) $\stackrel{+6}{\to }$ O (15) $\stackrel{+6}{\to }$ U (21) $\stackrel{+6}{\to }$ A ($27\equiv 1$)

Combining these, the missing letter cluster is NMMO.