Add a 'Download your tickets' button & the QR code to the EE4 thank you page. The hook in use adds the button & QR code before the overview, just after the confirmation order section. You will likely need to apply your own styles, possibly a container div and apply styles to that depending on how you want this to be displayed.

ee_display_download_tickets.php

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

function ee_display_download_tickets( $transaction ) {

	if ( $transaction instanceof EE_Transaction ) {

		$primary_reg = $transaction->primary_registration();

		if ( $primary_reg->is_approved() ) {

			$reg_url_link = $primary_reg->reg_url_link();

			$query_args = array(
				'ee' => 'ee-txn-tickets-approved-url',
				'token' => $reg_url_link
			);
			$ticket_url = add_query_arg($query_args, get_site_url());

			echo '<p class="jst-cntr"><a class="ee-button ee-roundish indented-text big-text" href="' . $ticket_url .'">Download your tickets!</a></p>';
            		
			echo '<p class="jst-cntr"><img src=http://chart.apis.google.com/chart?cht=qr&chs=200x200&chl=' . $reg_url_link . '&chld=H|0 alt="QR Code" /></p>';
		}

	}

}
add_action( 'AHEE__thank_you_page_overview_template__content', 'ee_display_download_tickets');