Edit in JSFiddle

<h5>容器宽度占满视窗宽度</h5>
<div class="container">
  <div class="element "></div>
  <div class="element "></div>
  <div class="element "></div>
  <div class="element "></div>
</div>

<h5>容器宽度占视窗宽度的80%</h5>
<div class="container cont2">
  <div class="element "></div>
  <div class="element "></div>
  <div class="element "></div>
  <div class="element "></div>
</div>

* {
  box-sizing: border-box;
}

h5 {
  background: #fff;
}
.container {
  width: 100%;
  background: #fff;
}

.container:after {
  display: block;
  width: 0;
  height: 0;
  content: '';
  clear:both;
}

.cont2 {
  width: 80%;
}

.element {
    width: 20%;
    float: left;
    padding-bottom: 15%; /* 4:3 */
    background: url(http://s2.zimgs.cn/ims?at=ori&kt=url&key=aHR0cDovL2FvMm8uYWxpY2RuLmNvbS9zdGF0aWMvb3NzL2FsaXl1bi1oei1zbS1vMm8wMS9hcHBzZXJ2ZXIvYnVjLzAzMGY3NGVkMzg4MzM2NTkwYmNlMmIwY2U5MTAyY2M1LmpwZw==&sign=yx:bVncy7Xz3AEIMEuDE9TtDRk5eLs=&tv=800_800&p=aHViYmxlX3VzZXJfaGVhZGVy:aHR0cDovL2FvMm8uYWxpY2RuLmNvbS9zdGF0aWMvb3NzL2FsaXl1bi1oei1zbS1vMm8wMS9hcHBzZXJ2ZXIvYnVjLzAzMGY3NGVkMzg4MzM2NTkwYmNlMmIwY2U5MTAyY2M1LmpwZw==) 50% 20% / cover no-repeat;
    background-color: #000;
}

.element:nth-child(n+2){
  margin-left: 5px;
}