Add the sample code below to your child theme's functions.php file or in a site specific plugin. Then change REPLACE ME to the new messaging that you would like to display for the Register Now button to your attendees/registrants.
<?php
//* Do NOT include the opening php tag
//* Adjust register now button text in Event Espresso 4
add_filter ('FHEE__EE_Ticket_Selector__display_ticket_selector_submit__btn_text', 'ee_register_now_button');
function ee_register_now_button() {
return 'REPLACE ME';
}
References:
http://eventespresso.com/topic/change-register-now-button/