Between 4 and 6 but not 5
The answer is:
4.5 (or any number between 4 and 6 that isn’t 5).
The most common intended answer is 4.5.
If you want a trick-style answer, it could also be:
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Any decimal between 4 and 6 except 5 (like 4.1, 4.9, 5.9, etc.)
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The number 4.999…
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A range (4 < x < 6, x ≠ 5)
But the simplest and most accepted answer is:
4.5
