Example migrated from Codex:

To hide some categories create a filter function in your functions.php file and hook it to the widget_categories_args filter. In this function, add the exclude parameter to the arguments list. Currently, in Version 3.3.2, the exclude parameter is not provided, but for future compatibility you should decide how you want to handle an already existing exclude parameter.

add_filter( 'widget_categories_args', 'widget_categories_args_filter', 10, 1 );

function widget_categories_args_filter( $cat_args ) {
	$exclude_arr = array( 4 );
	
	if( isset( $cat_args['exclude'] ) && !empty( $cat_args['exclude'] ) )
		$exclude_arr = array_unique( array_merge( explode( ',', $cat_args['exclude'] ), $exclude_arr ) );
	$cat_args['exclude'] = implode( ',', $exclude_arr );
	return $cat_args;
}