custom-404.php

// Stolen code, thanks Soma !
public function init() {
    $this->addHookAfter("ProcessPageView::pageNotFound", $this, "hook404Page");
}

public function hook404Page($event){
    header("HTTP/1.1 404 Page Not Found");
    $page = wire("pages")->get("/404/"); // get your 404 page
    $this->wire('page', $page);
    $event->return = $page->render(); // render and return that page
}