LESS is just a CSS preprocessor, it's your selector that could do with the improvement, we can combine that with LESS namespacing to have something like:
.A, .D
{
h4, a
{
color: #345e8a !important;
display: inline-block;
font-family: @font-family-sans-serif;
font-size: 17px !important;
}
}
In the above I've simply stripped out your .A .C and .A .B selectors. If you need those, then you'll probably end up with an equally messy solution, but slightly more readable:
.A .B, .A .C, .D
{
h4, a
{
color: #345e8a !important;
display: inline-block;
font-family: @font-family-sans-serif;
font-size: 17px !important;
}
}