Step 1: Concept
Identifying alphabetical progression for each letter position in the cluster.
Step 2: Analysis of first letters
O $\rightarrow$ P $\rightarrow$ Q $\rightarrow$ R (Consecutive letters)
The next letter after R is S.
Step 3: Analysis of second letters
K $\rightarrow$ L $\rightarrow$ M $\rightarrow$ N (Consecutive letters)
The next letter after N is O.
Step 4: Conclusion
The combined missing term is SO.
Final Answer: (4)