missing argument

Hej eksperter.
Nogen der ved hvorfor jeg får denne meddelse.

Warning: Missing argument 4 for add_to_database() in /home/administrator/www/test2/sample2.php on line 26
error:
Jeg har lavet et script der skal kunne indsende domæne navn og email til en mysql database.
Mit script er her
-----------------------------------------------
<?php

if ( isset( $domain ) && isset( $sex ) && isset( $domain ) )
    {
    //check user input here
    $db_error = "";
    $ret = add_to_database( $domain, $sex,$dberror );
    if ( ! $ret )
        print "error: $dberror<br>";
    else
        print "thank u very much";
    }
else    {
    write_form();
    }
   
   
function add_to_database( $domain, $sex, $mail, &$dberror )
    {
    $user = "root";
    $pass = "";
    $db = "sample";
    $link = mysql_connect( "localhost", $user, $pass );
   
    if ( ! $link )
        {
        $dberror ="couldn´t connect to database";
        return false;
        }
   
    if ( ! mysql_select_db( $db, $link ))
        {
        $dberror = mysql_error();
        return false;
        }
       
    $query = "INSERT INTO domains ( domain, sex, mail )
        values( '$domains', 'sex', '$sex' )";
        if ( ! mysql_query( $query, $link))
            {
            $dberror = mysql_error();
            return false;
            }
        return true;
        }
       
    function write_form()
        {
global $PHP_SELF;
print "<form action=\"$PHP_SELF\" method=\"POST\">\n";
print "<input type=\"text\" name=\"domain\"> ";
print "The domain you would like<p>\n";
print "<input TYPE=\"text\" name=\"mail\"> ";
print "Your mail address<p>\n";
print "<select name=\"sex\">\n";
print "\t<option value=\"F\"> Female\n";
print "\t<option value=\"M\"> Male\n";
print "</select>\n";
print "<input type=\"submit\" value=\"submit!\">\n</form>\n";
}
?>