<h1>Four items</h1>
<div class="container">
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
</div>

<h1>Six items</h1>

<div class="container">
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
  <div class="child"><img src="http://placehold.it/100/100"></div>
</div>

.container {
  width: 440px;
  display: flex;
}

.child {
  margin: 5px;
}

img {
  width: 100%;
}