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

.box {
  background: #000;
}
.box:nth-child(2) {
  background: red;
} 
.box:nth-child(3) {
  background: green;
}
.box:nth-child(4) {
  background: blue;
}
.grid {
  height: 100vh;
  display: grid;
  grid: auto / repeat(auto-fit,minmax(50%,1fr));
}