Bootstrap 4 navwalker - multilevel menu

1. Add this css

   ul.dropdown-menu li > ul.dropdown-menu{    left: 100%;    top: 0;}ul.dropdown-menu li:hover > ul.dropdown-menu, ul.dropdown-menu li:focus > ul.dropdown-menu{    display: block}

2. Remove this code && 0 === $depth from class-wp-bootstrap-navwalker.php

   original code:

   if ( isset( $args->has_children ) && $args->has_children && 0 === $depth && $args->depth > 1 ) {

   After editing:

   if ( isset( $args->has_children ) && $args->has_children && $args->depth > 1 ) {

Bulding simple menu list - here in user contributed notes (1st one) you can find code for build menu using menu array data, its simple and can be modified as you need.

Here is the code:

<?php// Intented to use bootstrap 3.// Location is like a 'primary'// After, you print menu just add create_bootstrap_menu("primary") in your preferred position;#add this function in your theme functions.phpfunction create_bootstrap_menu( $theme_location ) {if ( ($theme_location) && ($locations = get_nav_menu_locations()) && isset($locations[$theme_location]) ) {    $menu_list  = '<nav class="navbar navbar-default">' ."\n";    $menu_list .= '<div class="container-fluid">' ."\n";    $menu_list .= '<!-- Brand and toggle get grouped for better mobile display -->' ."\n";    $menu_list .= '<div class="navbar-header">' ."\n";    $menu_list .= '<button type="button" class="navbar-toggle collapsed" data-toggle="collapse" data-target="#bs-example-navbar-collapse-1" aria-expanded="false">' ."\n";    $menu_list .= '<span class="sr-only">Toggle navigation</span>' ."\n";    $menu_list .= '<span class="icon-bar"></span>' ."\n";    $menu_list .= '<span class="icon-bar"></span>' ."\n";    $menu_list .= '<span class="icon-bar"></span>' ."\n";    $menu_list .= '</button>' ."\n";    $menu_list .= '<a class="navbar-brand" href="' . home_url() . '">' . get_bloginfo( 'name' ) . '</a>';    $menu_list .= '</div>' ."\n";    $menu_list .= '<!-- Collect the nav links, forms, and other content for toggling -->';    $menu = get_term( $locations[$theme_location], 'nav_menu' );    $menu_items = wp_get_nav_menu_items($menu->term_id);    $menu_list .= '<div class="collapse navbar-collapse" id="bs-example-navbar-collapse-1">' ."\n";    $menu_list .= '<ul class="nav navbar-nav">' ."\n";    foreach( $menu_items as $menu_item ) {        if( $menu_item->menu_item_parent == 0 ) {            $parent = $menu_item->ID;            $menu_array = array();            foreach( $menu_items as $submenu ) {                if( $submenu->menu_item_parent == $parent ) {                    $bool = true;                    $menu_array[] = '<li><a href="' . $submenu->url . '">' . $submenu->title . '</a></li>' ."\n";                }            }            if( $bool == true && count( $menu_array ) > 0 ) {                $menu_list .= '<li class="dropdown">' ."\n";                $menu_list .= '<a href="#" class="dropdown-toggle" data-toggle="dropdown" role="button" aria-haspopup="true" aria-expanded="false">' . $menu_item->title . ' <span class="caret"></span></a>' ."\n";                $menu_list .= '<ul class="dropdown-menu">' ."\n";                $menu_list .= implode( "\n", $menu_array );                $menu_list .= '</ul>' ."\n";            } else {                $menu_list .= '<li>' ."\n";                $menu_list .= '<a href="' . $menu_item->url . '">' . $menu_item->title . '</a>' ."\n";            }        }        // end <li>        $menu_list .= '</li>' ."\n";    }    $menu_list .= '</ul>' ."\n";    $menu_list .= '</div>' ."\n";    $menu_list .= '</div><!-- /.container-fluid -->' ."\n";    $menu_list .= '</nav>' ."\n";} else {    $menu_list = '<!-- no menu defined in location "'.$theme_location.'" -->';}echo $menu_list;}?>