Add the event name as a CSS class to the body tag of the EE4 registration page. Useful for custom jQuery and/or CSS for specific events.

event_name_body_tag.php

<?php
//* Please do NOT include the opening php tag, except of course if you're starting with a blank file

add_filter( 'body_class', 'jf_ee_return_event_name_on_spco' );
function jf_ee_return_event_name_on_spco( $classes ){
    // change 'registration-checkout' to match your reg. page slug
    if (! is_page( 'registration-checkout' ) ){
        return $classes;
    }
    $events = array();
    $checkout = EE_Registry::instance()->SSN->checkout();
    if ( $checkout instanceof EE_Checkout ) {
        $transaction = $checkout->transaction;
        if ( $transaction instanceof EE_Transaction ) {
            foreach ( $transaction->registrations() as $registration ) {
                if ( $registration instanceof EE_Registration ) {
                    $event = $registration->event();
                    if ( $event instanceof EE_Event ) {
                        $classes[] = sanitize_title($event->name());
                    }
                }
            }   
        }
    }
    return $classes;
}