What's up with the number "four"?

I've never found an explanation for why we go from genitive singular to genitive plural:

Мне 1 год. Мне 2/3/4 года. Мне 5< лет.

I was looking at the Proto-Indo-European breakdown of numbers and the number "four" seems to have some significance there as well.

Also, (probably inexplicably), all the numbers between четыре and сорок end with ь. (пять, одиннадцать, двадцать...)

Thanks for shedding some light on this!