The correct way to spell 4 is f[u]ou[/u]r but for 40 it's f[u]o[/u]rty - why get rid of you?
[spoiler]Is there any etymological explanation for this?[/spoiler]
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I assume it's the same reason why any linguistic shortcuts are taken and why the American dialect exists: ease of use. The word 'for' already exists, so maybe that's why 'four' isn't contracted.