Add this to your WordPress website's custom functions plugin to make it so the return from PayPal page is https. Required: Event Espresso 4 and an SSL certificate installed for your domain.

add-to-functions.php

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

add_filter('_get_page_link','ee_paypal_return_https',10,2);

function ee_paypal_return_https($original_url,$post_id){
  if (class_exists('EE_Registry')){
    if(EE_Registry::instance()->CFG->core->thank_you_page_id == $post_id){
      $original_url = str_replace("http://","https://",$original_url);
    }
    return $original_url;
  }
}