#parentbox {
  display: flex;
  width: 600px;
  border: 1px solid;
  justify-content: center;
  flex-wrap: wrap;
}
#parentbox > div {
  height: 40px;
  margin: 5px;
  background-color: #CCCCCC;
}
#box-1 {
  width: 100px;
}
#box-2 {
  width: 100px;
}
#box-3 {
  width: 100px;
}
#box-4 {
  flex: 1 1 400px;
}