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function blogFilterH1($content) {
	if ( is_single() || is_archive() ) {
	$content = str_replace('<h1>', '<h4 style="margin-top:20px;">', $content);
	$content = str_replace('</h1>', '</h4>', $content);
	}
	return $content;
}

add_filter('the_content', 'blogFilterH1');