Adds a 'Go back to {event}' button to the registration-checkout page, change the styling to suit - http://take.ms/VXbHG

ee_back_to_event_button.php

<?php
/*
Plugin Name: EE Site Specific Plugin
Description: Add custom functions for Event Espresso 4 here.
*/
/* Begin Adding Functions Below This Line; Do not include an opening PHP tag as this sample code already includes one! */

//Add a 'Go back to the {event}' button below the registration form and payment selection.
function ee_display_return_to_event_on_spco(){
	
	//Pull in the current EE object.
	$checkout = EE_Registry::instance()->SSN->checkout();

		//Check we have an instance of the EE_Checkout object
		if ( $checkout instanceof EE_Checkout ) {

			//Pull the transaction object from checkout
			$transaction = $checkout->transaction;

			if ( $transaction instanceof EE_Transaction ) {

				//Pull the primary_registrant from the transaction.
				$primary_registrant =  $transaction->primary_registration();

				echo '<div class="jst-rght">
					<a class="ee-button ee-roundish indented-text" href="' .get_post_permalink($primary_registrant->event()->ID()). '">Go back to the "' . get_the_title($primary_registrant->event()->ID()) . '"</a>
				</div>';

		}

	}
}
add_action( 'AHEE__SPCO__after_registration_steps', 'ee_display_return_to_event_on_spco' );