I'm not exactly sure what layout you were going for over here, but you've specified that your 4th column spans 2 rows. Making this 1 row places everything in line:
.references {
  display: grid;
  grid-template-columns: 25% 25% 25% 25%;
  grid-template-rows: 25% 25% 25% 25%;
  padding: 1cm;
  height: 500px;
}
#subcontent {
  grid-column: 4;
  grid-row: 1;
}
<div class="references">
  <div>
    <h3>Lorem Ipsum</h3>
    <p>Nam volutpat pharetra velit in scelerisque. Etiam at dapibus justo. Ut porta enim massa, vel commodo nunc tincidunt eu. Sed sit amet eleifend turpis.</p>
  </div>
  <div>
    <h3>Lorem Ipsum</h3>
    <p>Nam volutpat pharetra velit in scelerisque. Etiam at dapibus justo. Ut porta enim massa, vel commodo nunc tincidunt eu. Sed sit amet eleifend turpis.</p>
  </div>
  <div>
    <h3>Lorem Ipsum</h3>
    <p>Nam volutpat pharetra velit in scelerisque. Etiam at dapibus justo. Ut porta enim massa, vel commodo nunc tincidunt eu. Sed sit amet eleifend turpis.</p>
  </div>
  <div id="subcontent">
    <h3>Lorem Ipsum</h3>
    <p>Nam volutpat pharetra velit in scelerisque. Etiam at dapibus justo. Ut porta enim massa, vel commodo nunc tincidunt eu. Sed sit amet eleifend turpis.</p>
  </div>
</div>