Actually, you can't pick when to forfeit (though if I made the rules, you would be able to). The forfeit has to go last.
In order to forfeit while still playing 4 players that add up to 20 or 21, you have to have a roster that would allow you to meet the 23 rule, even if one of the needed players is absent. Otherwise, you have to play four players whose skill level adds up to 19.[/
Unless its been changed since 2011 you can forfeit individual matches and it doesn't have to be the last match. There are two ways which you can do it, one of them the balls do not have to be broken, the other you wait until the first rack is broken. If you forfeit and do not break the first rack then the forfeiting team loses their player for the match, the non forfeiting team DOES NOT lose their player. However the skill level(s) still counts against the 23 rule.