There have been more than four previous editions

From Traveller5

Previous Editions Of Traveller

Classic Traveller (CT). The original edition of Traveller published by GDW, released in 1977, and revised in 1981. The core rulebooks were intended as generic science-fiction, and the development of the Third Imperium setting came through adventures and supplements.

MegaTraveller (MT). The second edition of Traveller published by GDW, released in 1987, introduced a unified task structure and a fully developed skill system, and advanced the Imperium setting into the Rebellion era.

Traveller: The New Era (TNE). The third and last edition of Traveller published by GDW, released in 1993, changed the game mechanics to be compatible with the GDW "House System" of RPG rules. The setting is one of recovery after an interstellar collapse.

Marc Miller's Traveller (T4). Created as a successor to the GDW editions, published by Imperium Games and released in 1996, and rolling back the setting to the founding years of the Third Imperium.

Traveller5 (T5). The fifth of the direct line of editions of the Traveller game system, intended as the ultimate science-fiction role-playing system, and capable of managing situation across a variety of eras and technology levels.

GURPS Traveller.

HERO Traveller.

Action Traveller.

Mongoose Traveller.

D20 Traveller.