All these numbers went through similar changes over time, PB, so the fact that they are not identical in 'shape' is not wholly surprising. In Old English, for example, we had threeotynne...by the 14th century, thritten...and by the 17th century, thirteen. Note how the vowel 'i' remained IN FRONT of the consonant 'r' for about three quarters of a millennium, before moving to its present position.
Re forty, the basic word four originally had an 'ow' sound, as it still does in parts of Scotland, "What time is it?" "It's half past fower." When it took on the simpler 'oh' sound in English speech, the 'u' was really superfluous and so disappeared from the spelling.