How can I add headings using the Rich Text field How can I add the functionality for my clients to make text not just bold or italic, but have h2, h3, h4 headings too?
You can modify Craft's default Redactor configs stored in craft/config/redactor or add custom config files to this directory. Then select the config of choice for each rich text field individually in the field settings.

To use "block styles" in the editor, add the formatting button to the toolbar with this config:

{
    buttons: ['formatting', 'bold', 'italic']
}


You can also modify the formatting dropdown, if you want to restrict the block styles available to your client:

{
    formattingTags: ['p', 'h2', 'h3', 'h4']
}


.

Edit:

Just noticed that you already have the button in your toolbar, it's the second one from the left :D