If you look at your toolbar, to the right of the central filepath box you should see one or more icons. Each of these is an Editor that 4Pane has detected on your system.
If you click one of these icons, 4Pane launches its Editor. More usefully, if you drag one or more files onto the icon, the Editor is launched with those files open in it. Some Editors (e.g. kwrite) can open only one file at a time; others (gedit) several.
An Editor can be any application. Though most will be actual editors like kwrite or gedit, good examples of non-editor Editors are LibreOffice and Firefox (both of which 4Pane will automatically add if it detects them). These are applications that will accept files, but there's nothing to stop you creating an icon for an application that doesn't.
You can add a new Editor, or delete one, from Options > Configure 4Pane, in the Display > Misc section.