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]
Just the way things work out. Why do we say [i]twenty?[/i] What significance does [i]twe[/i] hold to the number two?