Much like the Star Wars Expanded Universe (or, as the hipsters now call it, Legends) the best “source” for your IP-based RPG often is non-canonical. Memory Alpha tells us about The Four Years War:
This sourcebook was released as a scenario for Star Trek: The Role Playing Game, and used references from several novels and the Spaceflight Chronology (which differs from the more accepted and canon material in the Star Trek Chronology and later Star Trek: The Next Generation– and Star Trek: Enterprise-era productions). Introduced as “a concise compilation of library information on the conflict between the UFP and Klingons, stardates 1/9409 though 1/9806 (in the 2250s).” This was originally released as a set with book #2218, Return to Axanar.
It’s really a shame that The Four Years War (FASA Corp, 1986) is not Star Trek canon because the book is an awesome piece of gaming literature; a wonderful marriage of role-playing sourcebook and wargame campaign.
It’s sometimes hard to imagine that Star Trek was not always the powerhouse IP that it is today. Growing up in the late 1970’s all one got was reruns of the original series confined to syndicated television on UHF channels (if you don’t know what I mean by that, well, you just showed your age). In the gaming world, however, Star Trek was an IP that even small game companies could go after. In the wargame world it was Task Force Games with Star Fleet Battles (Task Force Games, 1977+); in the RPG world it was FASA Corporation and their Star Trek: The Role Playing Game (FASA Corporation, 1982-1989). I played both Star Fleet Battles (SFB) and Star Trek: The Role Playing Game (STRPG). As a wargamer, I got heavily into SFB but I didn’t go as deep into STRPG in part because I was a heavy Traveller RPG player.
At the time the first SFB and STRPG came out, the “canon” of Star Trek was still ill-defined. SFB based itself on The Original Series, Animated Trek, and most importantly Franz Josef’s Star Fleet Technical Manual. Unlike SFB, the authors of STRPG were able to incorporate elements of Star Trek: The Motion Picture (1979), Star Trek II: The Wrath of Khan (1982), and eventually even more into their game while still creating new and original content. They created content until the great Paramount evil took back control of their IP. The designer’s notes in The Klingons Sourcebook actually tells us a great deal about the latitude the game authors had and what they were able to do with it.
A great example of this latitude and new content is The Four Years War. Reading through this collection of library entries, one learns not only about the military timeline, equipment, and tactics of the war but also the social, political, and economic factors of the day. The book is chock-full of inspiration not only for battles or military campaigns, but also background and plot seeds for a really interesting RPG adventure. There is enough here to ensure that the adventure doesn’t have to be a military campaign; there are many different seeds here to make something far more interesting.
The book is broken up into seven major sections. Within each are several “entries” that deliver the informationin small, really “bite-size” fragments. I especially like that competing viewpoints are included, like “UFB Military Background” and “UFP Military Background – A Criticism” or “Klingon Strategy” and “Klingon Strategy – A New Theory.” These multiple viewpoints engage the GM to design scenarios without a straitjacket interpretation of the situation already laid out by a lawyer from Southern California.
Not only does The Four Years War provide setting material for Star Trek: The Role Playing Game but it feeds other portions of the FASA STRPG product line. Entries like “Consequences for Military Shipbuilding Theory” help one design new ships in the game. If one has the companion Star Trek: Starship Tactical Combat Simulator (FASA Corp, 1983) then the entire section on Starship Tactics is for you.
Resurrected Starships on YouTube has made a video explanation of The Four Years War. Check it out and tell me what you think!
Feature image Return to Axanar and The Four Years War set from waynesbooks.net
Rocky Mountain Navy Gamer 
i actually remember playing the RPG and it was a lot of fun. I remember reading through all of the adventures we had (that Return to Axenar cover really prompted a memory).
I don’t know how much we played it, but the materials themselves were great!
Not familiar with the 4-Years-War book, though.
Looks interesting.