<div class="container">
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
  <div class="box"></div>
</div>

body {
  display: flex;
  justify-content: center;
  align-items: center;
  min-height: 100vh;
  padding: 0;
  margin: 0;
}

.container {
  max-width: 600px;
  width: 100%;
  display: flex;
  justify-content: center;
  align-items: center;
  flex-wrap: wrap;
  border-top: 1px solid grey;
  border-left: 1px solid grey;
}

.box {
  width: 25%;
  height: 100px;
  border-bottom: 1px solid grey;
  border-right: 1px solid grey;
  box-sizing: border-box;
}