You can put this code snippet on your site and it will disable the EE4 registration form field inputs for First Name, Last Name, and Email if the user is logged into the website. Useful with the WP User add-on.

read_only_questions.php

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

function ee_custom_disable_specific_questions() {
    if ( is_user_logged_in() ) {
        wp_add_inline_script( 
            'ee_form_section_validation',
            'jQuery( document ).ready(function($) {
                $("input[id$=-fname],input[id$=-lname],input[id$=-email]").prop( "readonly", true );
            });'
        );
    }
}
add_action( 'wp_enqueue_scripts', 'ee_custom_disable_specific_questions', 11 );