<?php
$images_dir = '/home/xleo/tests/webm/images';

if (chdir($images_dir)) {
    $files = scandir('.');
    foreach ($files as $oldname) {
        if (preg_match('/\d{4}.jpg$/', $oldname)) {
            $newname = preg_replace('/^.*(\d{4}.jpg)$/', '\1', $oldname);
            rename($oldname, $newname);
        }
    }
}