fixed-fluid-items.css

.container {
  width: 500px;
  border: 5px solid black;
  display: flex;
}
.item {
  flex: 1 1 auto;
  width: 100px;
  height: 100px;
}
.one {
  background: #B5AC01;
}
.two {
  height: 120px;
  background: #ECBA09;
}
.three {
  background: #E86E1C;
}
.four {
  flex: 0 0 auto;
  height: 140px;
  background: #D41E45;
}