Found the way by myself:
add_filter('acf/get_valid_field/type=wysiwyg', 'my_get_valid_field_post_content');
function my_get_valid_field_post_content( $field )
{
if ( $field['name'] == 'post_content' )
$field['type'] = 'textarea';
return $field;
}
But please reply to part 2 of my first message about difference between version 4 and 5 and how they work with post_content.
Thanks in advance!