Space Quest 4 was released in 1991. There is a re-release is 1992 that feature full speech support, but unfortunately I don't have a copy of that. Space Quest 4 is the first Sierra game that support 256-color VGA graphics and point-and-click interface.
In this installment, Roger embarks on a time-travel adventure through Space Quest games both past and future.
An infomorph of reborn Sludge Vohaul from Space Quest XII chases Roger through time in an attempt to finally kill him. Roger also visits Space Quest X: Latex Babes of Estros and Space Quest I; in the latter.
When go back to Space Quest I, the graphics and music revert to the style of the original game and Roger is threatened by a group of monochromatic bikers who consider Roger's 256 colors pretentious.
An easter egg in this game is accessed by entering the code in the time capsule as the top row of buttons in order from left to right. This transports Roger to the volcanic planet of Ortega in Space Quest 3. Unfortunately, this time Roger is not wearing any thermal underwear, so if he steps out of the capsule, he melts.
There is also a mini-game in Space Quest 4, Ms. Astro Chicken: Flight of the Pullet. Ms. Astro Chicken appears within the context of the game in the form of an arcade game. It is a sequel of sorts to Astro Chicken in Space Quest 3.
The gameplay and storyline of Space Quest 4 is as good as the previous instalments. Even though it change the interface to point-and-click system, the feel of the series is still well maintained.
You can get Space Quest 4 here.