docs: underline level four heading links too

Author: luisadame

docs/src/components/markdown.module.css

@@ -156,7 +156,9 @@
   @apply text-xl;
 }
 
-.markdown > h2 > a, .markdown > h3 > a {
+.markdown > h2 > a,
+.markdown > h3 > a,
+.markdown > h4 > a {
   @apply underline;
 }