I learned this when I was a child; it's adapted from a story in Greek mythology.
An infant crawls on all fours--in the 'morning' of life
An adult walks on two legs--'in the midday/afternoon of life'
An elderly person walks on two legs and leans on a cane=3 'legs'.
a human 4 in then morning morning means when there a baby and they crawl on all fours 2 in the afternoon which mean when there and adult and walking on both legs and at night when there old they use a cane