#parentbox {
  display: flex;
  width: 600px;
  height: 200px;
  border: 1px solid;
  align-items: stretch;
}
#parentbox > div {
  margin: 5px;
  background-color: #CCCCCC;
}
#box-1 {
  flex: 1;
}
#box-2 {
  flex: 1;
}
#box-3 {
  flex: 1;
}
#box-4 {
  flex: 1;
}