We all know that the cardinal and ordinal numbers 4 and 4th are spelled ‘four’ and “fourth” respectively. Then we have 14 and 14th which are spelled “fourteen” and “fourteenth”. Yet the numbers 40 and 40th are spelled without the letter ‘u’, as in “forty” and “fortieth”.
It is said that Old English and Middle English spelling was phonetic but over time letters were either added or removed from words to resemble more closely their Latin origins; e.g., dette was replaced by ‘debt’ (L. debitum). And as customs in pronunciation changed so did the spelling, for instance ‘drink’ used to be drincan in Old English.
With that in mind, it's not a surprise when the letter ‘u’ in fourty became silent, it was dropped.
The logical Middle English relic fourty, …, lasted until the 18th century, when for reasons unknown it fell out of use. Sometimes that's just how it goes in English.
However, the same spelling change did not happen to “four” /fɔː/, and "fourteen" /fɔːˈtiːn/ or /ˈfɔːtiːn/, despite the ‘u’ being also silent.
Is there an explanation for this discrepancy? Why did the spelling change for “forty“ but not for numbers “four” and "fourteen”?