The word four has four letters. In the English language there is no other number whose number of letters is equal to its value

While it's true that 'four' has four letters, the claim that it's the only number with this property is false. Several other numbers in English have letter counts equal to their value, including 'three' (5 letters - wait, that's 5, not 3). Let me reconsider - actually, in English, 'four' may indeed be one of the very few or only numbers with this property.