WordPress at a glance
filter is not described

mce_buttons_4 filter-hook . WP 2.5.0

Filters the fourth-row list of TinyMCE buttons (Visual tab).

Usage

add_filter( 'mce_buttons_4', 'filter_function_name_4204', 10, 2 );
function filter_function_name_4204( $buttons, $editor_id ){
	// filter...

	return $buttons;
}
$buttons(array)
Fourth-row list of buttons.
$editor_id(string)
Unique editor identifier, e.g. 'content'.

Where the hook is called

_WP_Editors::editor_settings()
wp_tinymce_inline_scripts()
wp-includes/class-wp-editor.php 628
$mce_buttons_4 = apply_filters( 'mce_buttons_4', array(), $editor_id );
wp-includes/script-loader.php 674
$toolbar4 = apply_filters( 'mce_buttons_4', array(), 'classic-block' );

Where the hook is used (in WP core)

Does not used.