<div class="col">
  <div class="section">
  </div>
   <div class="section">
  </div>
   <div class="section">
  </div>
   <div class="section">
  </div>
</div>

.col {
  width: 100%;
  display: flex;
  flex-wrap: wrap;
}

.section {
  margin: 10px 0px 0px 10px;
  min-width: 250px;
  height: 400px;
  background-color: gray;
  flex: auto;
}