add a sponsor section to EE4 Single Page Checkout, requires the Advanced Custom Fields plugin

sponsors.php

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

function ee_reg_steps_area() {
    $checkout = EE_Registry::instance()->SSN->checkout();
    if ( $checkout instanceof EE_Checkout ) {
        $transaction = $checkout->transaction;
        if ( $transaction instanceof EE_Transaction ) {
            $registrations = $transaction->registrations();
            $registration = reset( $registrations );
            if ( $registration instanceof EE_Registration ) {
                $event = $registration->event();
                if ( $event instanceof EE_Event ) {
                    ?>
                    <div class="sponsordetails">
                    <h3><span class="dashicons dashicons-heart"></span>Event Sponsor</h3>
                    <h4><img align="right" src="<?php the_field( 'sponsor_logo_image', $event->ID() ); ?>">
                        <strong><a href="<?php the_field( 'sponsor_website', $event->ID() ); ?>">
                            <?php the_field( 'sponsor_company', $event->ID() ); ?>
                        </a></strong>
                    </h4>
                    <?php the_field( 'sponsor_bio', $event->ID() ); ?>
                    </div>
                    <?php
                }
            }
        }
    }
}