Based on the English alphabetical order, three of the following four letter-clusters are alike in a certain way and thus form a group. Which letter-cluster does not belong to that group?
(Note: The odd letter-cluster is not based on the number of consonants/vowels or their position in the letter-cluster.)

Let's find the difference between the alphabetical positional values of the letters in each cluster:
1. CGE:
* C (3) $\stackrel{+4}{\to }$ G (7)
* G (7) $\stackrel{-2}{\to }$ E (5)
Pattern: $+4,-2$

2. MQO:
* M (13) $\stackrel{+4}{\to }$ Q (17)
* Q (17) $\stackrel{-2}{\to }$ O (15)
Pattern: $+4,-2$

3. GKI:
* G (7) $\stackrel{+4}{\to }$ K (11)
* K (11) $\stackrel{-2}{\to }$ I (9)
Pattern: $+4,-2$

4. KON:
* K (11) $\stackrel{+4}{\to }$ O (15)
* O (15) $\stackrel{-1}{\to }$ N (14)
Pattern: $+4,-1$

All clusters follow the pattern $+4,-2$ except for KON, which follows $+4,-1$. Therefore, KON is the odd one out.