I've a script running without problems under 4.0.0, using 4.0.x (x > 0) results in the same problem. The connection is established, but when passing it's descriptor as an argument, to any other pgsql function, it warns, that it's no pg connection identifier. The script could look like this:
$db = db_connect($db_server_name, $db_server_port, $db_name,                  $db_user, $db_password);
if (!$db) {
  exit();
}
// next line produces the warning
$res = db_query($db, "select * from table");

Called funcions are defined in http://wp-klenci.domazlice-net.cz/dp/php/lib/1st_level/pgsql.phps as a simple wrapper to pg functions.

Ondrej Svetlik