[jennyfur]

master error is correct, you'd need:

sv_privateclients 4
sv_privatepassword whatever

in your server config (and by the way, the maxclients command is sv_maxclients, not ui_maxclients. it's meant to be the maximum total number of clients you want the server to have....which you mentioned was 20). to connect to those reserved spots from a client, you'd just need to type password whatever in the console before you connect, or make a config to do it automatically (much like the rcon config explained in bull's dedicated server and rcon setup guide here on allied assault).