I find that if you lead with the thumb-as-1 - then I can't use my little finger held down for 4.
I naturally use my thumb to hold down the other fingers leading with index-as-1.
I'm not sure what the thumb-as-1 does for 4 though.
Since the thumb is the first digit on the hand (depending on the way you go) I can see it being a logical choice - but not necessarily the most convenient once you get to 4.