1) It’s tag__in
(two underscores) 2) get_the_tags()
returns an array of objects, you need IDs:
if ( $the_tags = get_the_tags() ) {
// https://developer.wordpress.org/reference/functions/wp_list_pluck/
$the_tags = wp_list_pluck( $the_tags, 'term_id' );
}
$rel_pst = get_posts( array(
'tag__in' => $the_tags,
'exclude' => $post->ID,
'post_per_page' => 4,
'ignore_sticky_posts' => true,
'post_type' => array(
'ms',
'gnd',
'events',
'news_article',
'opinions',
'projects',
'tenders',
'videos',
'products',
),
));
if ( $rel_pst ) {
// Do your thing
}