Instead of using install_guestbook.php try to run this code in your phpmyadmin, but please make a backup before because I'm not sure it's correct!
INSERT INTO `4images_settings` (`setting_name`, `setting_value`) VALUES ('guestbook_view', '1');
INSERT INTO `4images_settings` (`setting_name`, `setting_value`) VALUES ('user_delete_guestbook', '0');
INSERT INTO `4images_settings` (`setting_name`, `setting_value`) VALUES ('guestbook_post', '1');
INSERT INTO `4images_settings` (`setting_name`, `setting_value`) VALUES ('user_edit_guestbook', '0');
CREATE TABLE `4images_guestbook` (
`comment_id` mediumint(8) NOT NULL auto_increment,
`user_name` varchar(100) NOT NULL default '',
`comment_site` varchar(255) NOT NULL default '',
`comment_date` int(10) unsigned NOT NULL default '0',
`comment_text` text NOT NULL,
`comment_ip` varchar(20) NOT NULL default '',
PRIMARY KEY (`comment_id`),
KEY `comment_date` (`comment_date`)
) TYPE=MyISAM;