You easily add an H4 tag in the “Visual” version of the editor (the tab on the upper right corner of the editor) by selecting the text you want to be the subheader at the top of the rich text editor there is a buttom at the end of the first row of icons called the “Kitchen Sink”, click this to expose all the options the editor has available, now select the drop down hich likely says “Paragraph” and change it to “Heading 4”.
If you are on the “Text” version of the editor, you would add the <h4></h4> tags around your subheading.