As you already know that code provided on the old forum does not work anymore, that is because that code is for the old version of X, and it breaks if its use for the new version, for this very reason, regretfully we cannot simply provide any customization/modification support for any element.
While that is outside the scope of support, I could point you in the right direction with the understanding that it would ultimately be your responsibility to take it from here.
You can find the new Accordion Item function code on accordion.php file under
\wp-content\plugins\cornerstone\includes\shortcodes\ directory. Copy that code to your child theme's functions.php file, and modify it.
If you only want the accordion header to be styled like h4, then please use the V2 (new) Accordion element instead, it has a lot of styling option than the Classic Accordion element.
Thank you for understanding,