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>