From looking at their styles h4-h6 are all the same.
You can personalise how they display in the viewer pane by adding some entries to your userstyle.css file. Just pick the font sizes that suit your needs.
h1 {font-size: 30px;}
h2 {font-size: 25px;}
h3 {font-size: 20px;}
h4 {font-size: 16px;}
h5 {font-size: 12px;}
h6 {font-size: 8px;}
