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

.container {
  display: flex;
  flex-wrap: wrap;  /* Allows items to wrap onto the next line */
  justify-content: space-between;
}

.item {
  flex: 1 1 200px;  /* Each item has a base size of 200px and can grow/shrink */
  margin: 10px;
  padding: 20px;
  background-color: #f0f0f0;
  text-align: center;
}