SOURCE

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

.box {
  width: 600px;
  height: 600px;
  background: red;
  display: flex;
  flex-direction: row;
  flex-wrap: wrap;
  /* align-content: flex-start; */
  justify-content: space-between
}
.item {
  width: 250px;
  height: 100px;
  background: blue;
}