I'm guessing 's' because it's one of the most common consonants in English. It could appear at the beginning of the word (like "s___e_"), as part of a common ending (like "___es"), or elsewhere in the word. 5-letter words with 'e' as the 4th letter might include words like "speak", "sweet", "shear", etc. The letter 's' appears frequently in English words and would give us valuable information regardless of whether it's in the word or not.