Final answer:
The 4-letter preposition that starts many fairy tales is 'Once.'
Step-by-step explanation:
The 4-letter preposition in the 4-word phrase that starts many a fairy tale is D) Once. In fairy tales, the phrase often begins with 'Once upon a time,' which sets the story in the past and creates a sense of magical or fantastical elements. For example, 'Once upon a time, there was a princess who lived in a castle.'