I think it would be kind of fun if 4 players with companions could meet up and complete a quest, or a theme park.

If there ever was to be a multiplayer Fallout, it should be a completely separate game dedicated solely for that purpose and developed by a company specialized for that sort of thing, and have generally nothing to do with the rest of the series other than sharing the setting. And no, that - in my opinion - shouldn't be the "next" Fallout.

Please use the http://www.gamesas.com/index.php?/topic/1112031-fallout-multiplayer-discussion-thread/ for all multiplayer discussion.