Hi,
If the payment with PayPal works and confirms the orders then HikaShop is configured properly and working fine.
If the problem is with the Bizum payment plugin then the first thing to do is to check with the developer of the plugin. Maybe they have made a new version of the plugin for Joomla 4 and you need to update the plugin ? Or maybe there is a problem with Joomla 4 in the plugin and they're not aware of it, and thus they'll be able to fix it and provide you a new version of the plugin.
Now, if you want me to look into it, I can, but since that's outside normal user support for HikaShop itself, I'll have to charge you for the time spent on it. If you're interested, please go through our contact form to provide the backend and FTP credentials and any other information you might have useful to this (and include a link to this thread, for reference):
www.hikashop.com/support/contact-us.html