Police Quest IV: Open Season
When looking at your desk in your office, look at the desk to your left for a reference to Ken Williams, founder of Sierra On-Line (who indeed moved to Seattle when Sierra opened its new headquarters there).
The computer monitor in Parker Center is of the brand "BrightStar", which is the name of Sierra's daughter company which developed edutainment titles like the Talking Tutor series. Also, you log in on server "Sonny", a reference to Sonny Bonds, the protagonist of Police Quest 1 through 3, designed by Jim Walls.
One of the gangs in the computer database is called the "Red Dogs", stating that two of its members, Charlie and Slater are hiding in the Sierra Nevada mountains. This is a reference to Slater & Charlie Go Camping, developed by Sierra's daughter company Red Dog Studios.
When using the elevator at Parker Center to floors 2 or 5, there's a queue of people blocking your path, presumably Sierra employees. The people standing there are randomly picked each time your reach the floor, but one of them is undeniably Scott Murphy, even referred to as an Andromedan. On floor 6, you'll find Daryl F. Gates, each time telling you a different line, including the explicit "Tell the media to go f' themselves". On the morning of day 2 (the morning after the alley crime scene), there will be a janitor instead.
When you're not in the company of Sam and Julie, you can play the arcade game in the Short Stop Bar. It's titled "Stroids, another fine adventure from the Sierra family of games". At the end of the game you'll see the high scores, containing both names of Sierra game characters (Larry Laffer, Sony Bonds) and Sierra employees (Chris Carr, Dan Woolard, Daryl Gates and Lyle Lovett).
There is a bottle of Jameson - real Irish Whiskey - on the counter of the Bitty Kitty Club. Did Sierra promote an actually existing alcoholic beverage in this game?
Use the hand icon on the computer in Luella Parker's office to get a reference to "Space Quest V: The Final Frontier". The real title of Space Quest V, released a bit before Police Quest 4, is actually "Space Quest V: The Next Mutation". Space Quest VI on the other hand was titled "The Spinal Frontier" but wasn't released until late 1995. Weird uh?