It is not actually clear from the wording of the Rulebook whether the post-USO one must be counted, or if it just must be played. Presumably one of the players who must play 4 500s will be able to count the highest 4, as long as they did play one post USO.

Lord alone knows what happens if any of them actually do not play one after the USO. Make them count a zero? Just make them do PR work as a penalty? Anybody's guess.