Well, what you're describing is reminiscent of the Files Magazine publication The Star Trek That Almost Was
, which described several rejected story treatments and outlines for TOS, though not specifically for a fourth season, just generally. But it was written by John Peel, not two authors associated with the show.
Oh, wait -- you could be thinking of Star Trek Phase II: The Lost Series
by Judith & Garfield Reeves-Stevens. It was about the abortive '70s revival series that ended up becoming Star Trek: The Motion Picture
, and it included various plot outlines developed for the series. But the Reeves-Stevenses weren't involved with the original series, although they were story editors on the final season of Enterprise
well after the book was published, and have written multiple ST novels including co-writing the William Shatner novels.