Add a 'Download your tickets' button to the EE4 thank you page. The hook in use adds the button above the overview, just under 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 '<a class="ee-button ee-roundish indented-text big-text" href="' . $ticket_url .'">Download your tickets!</a>';

		}

	}

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