Leisure Suit Larry 4 is a computer action-arcade game of mythical proportions.
Depending on the time sector and version of Roger's past this game may have a number of different subtitles or files on the disk including Leisure Suit Larry vs. The Women of Mars and Bring Me the Head of Roger Wilco--Only He Can Heal My Broken Heart.
Leisure Suit Larry 4 was said to have been designed and written by Sierra Online (though ScumSoft may have made their own version or stolen the code, or had publishing/distribution rights in the future). Unfortunately, the floppies upon which the one and only copy of the game was stored, were lost. Unbeknownst to the employees of Sierra Online, the disks were apparently stolen by an agent of Sludge Vohaul, who subsequently used them to store his consciousness upon his demise in Space Quest 2.
Later, the floppies were found floating in space by a Pacoid crew of scientists in the Xenon Super Computer Project. When the game was installed on the computer, Vohaul's consciousness was unleashed on the planet, and the events leading up to Space Quest 12 began.
The sad story of these events leading up to the fall of Xenon were relayed to Roger Wilco during his adventures in Space Quest 4 by a hologram of Professor Lloyd, one of the members of the original Xenon Super Computer team.
While the Leisure Suit Larry Series was indeed created by Sierra Online, there never was a Leisure Suit Larry 4. According to Al Lowe (the Larry series creator), Leisure Suit Larry 4 had originally been intended as an early online game. It was cancelled when another Sierra Online project ("The Sierra Network") turned into a money pit. When Lowe began work on the next single player Larry game, he decided to call it Leisure Suite Larry 5, skipping 4 altogether. From then on, "Leisure Suit Larry 4: The Missing Floppies" became a Sierra in-joke.
Al Lowe's site has more information.