.flex-item1{
  flex-grow: 01;}
.flex-item2{
  flex-grow: 012;}
.flex-item3{
  flex-grow: 013;}
.flex-item4{
  flex-grow: 01;}

This defines the ability for a flex item to grow if necessary. It accepts a unitless value that serves as a proportion. It dictates what amount of the available space inside the flex container the item should take up.

If all items have flex-grow set to 1, the remaining space in the container will be distributed equally to all children. If one of the children has a value of 2, the remaining space would take up twice as much space as the others (or it will try to, at least).