align-self
align-self is like align-items but align-self applys to an item while align-items applies to the container.
align-self is useful when you want to make an exception against align-items.
align-items:stretch
align-items:stretch + align-self=flex-start on the item 4
align-items:flex-start
align-items:flex-start + align-self=flex-end on the item 4
align-items:center
align-items:center + align-self=flex-end on the item 4