I'm a bit uncertain about the design you had in mind for this section, but it seems like you indicated that the 4th column should span 2 rows. If we adjust this to have only 1 row, everything will align properly:
.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>