Index: ms-functions.php
===================================================================
--- ms-functions.php	(revision 19980)
+++ ms-functions.php	(working copy)
@@ -458,8 +458,8 @@
 	if ( is_email_address_unsafe( $user_email ) )
 		$errors->add('user_email',  __('You cannot use that email address to signup. We are having problems with them blocking some of our email. Please use another email provider.'));
 
-	if ( strlen( $user_name ) < 4 )
-		$errors->add('user_name',  __('Username must be at least 4 characters'));
+	if ( strlen( $user_name ) < apply_filters( 'wpmu_user_name_length', 4 ) )
+		$errors->add( 'user_name',  __( sprintf( 'Username must be at least %d characters', apply_filters( 'wpmu_user_name_length', 4 ) ) ) );
 
 	if ( strpos( ' ' . $user_name, '_' ) != false )
 		$errors->add( 'user_name', __( 'Sorry, usernames may not contain the character &#8220;_&#8221;!' ) );