Very strange. I see the login page is ok. There could be different reasons:
- your PHP lacks mysql support (install or enable php-mysql extension, see php.ini)
- a web-server level filter (like apache's mod_security) doesn't like install page and redirects you to 404
To debug, try to insert "return;" into the following places into install/index.php (one by one in turn).
Please, report if it starts showing the page.
function check_status()
{
   global $page, $webimroot, $settings, $dbversion;
   $page['done'][] = getlocal2("install.0.php", array(phpversion()));
         /// <<HERE>>
   if (!check_webimroot()) {
      return;
   }
         /// <<HERE>>
   if (!check_files()) {
      return;
   }
         /// <<HERE>>, etc.