I came across this enigmatic one-liner:
How can four be half of five?
Can you figure it out?
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Another (kind of linguistic) answer:
Since the word half, at least historically, means "one of two approximately equal parts" (for example, Merriam-Webster gives the following example: “Which half do you want?” “I'll take the smaller half.”), we can say the following: Four is the half of five, as long as one is the other half of it!