Flex Example

HTML

<div class="container">
  <div>Div 1</div>
  <div>Div 2</div>
  <div>Div 3</div>
  <div>Div 4</div>
</div>

CSS

html,
body,
.container {
  height: 100%;
}

.container {
  border: 1px solid green;
  display: flex;
  flex-wrap: wrap;
}

.container > div {
  border: 1px solid black;
  background: #ececec;
  flex: 1;
}

.container > div:nth-of-type(1) {
  flex: 1 1 100%;
}

.container > div:nth-of-type(4) {
  flex: 1 1 100%;
}