Whilst on a mission with Packie, if you position the camera properly you can look inside his head - inside there is a box with a picture of a brain on it.
In the North of Alderney, where you make the jump in the mission Out of Commission, there is a person doing tai-chi on top of a high rock which is impossible to get to without a helicopter.
If you look at the lake in Middle Park by map or helicopter, you'll notice it is in the shape of an alien.
While driving the Mr. Tasty, you can trigger songs to play by clicking the horn. One of these songs is the song from the opening credits of GTA4. While this song is playing, call ZiT and they will text you back with "ZiT! Spotted GTA IV Theme". This is a cheat that gives the player health, armor and ammo, but also disables some achievements.
If you take a helicopter ride from Brucie Kibbutz, or you go for a helitour, there's an unsettling warning message about the helicopter inside the cockpit that you can see if you look in first person mode that says "This helicopter might crash but we really don't care if it does, refer to flight manual for help."
At the Firefly Projects, there is a park with a swingset. If you drive backwards (or forwards with some vehicles; Voodoo) onto it, the swingset will catapult the vehicle you're in high in the air. This is likely a bug in the game's physics simulation.
If you are driving a car or bike at an incredible speed, go to your cellphone and go to camera and when you select camera, no matter at what speed you are going your vehicle will come to an instant stop.
Behind an old, rundown mansion in Westdyke, Alderney at the end of the dirt roads, there is the hidden, extremely rare Sultan RS parked in some bushes. This is practically the only way to find the car ingame. The only other way to obtain it is if the game has it stored in the temporary memory, and automatically spawns it parked and driving around the streets.
References to previous games
On the Mission "First Date", If you selected "Try your Luck", Niko will ask if he can come in for coffee, then Michelle will say "Yeah, for 20 million dollars". This is a reference to the "Hot Coffee" controversy which describes the lawsuit.
Descending down the stairs of your first apartment in Hove Beach, you may spot some graffiti referring to past GTA protagonists Claude, Tommy, Carl (CJ), Toni, and Vic suggesting they have passed away. This can also be seen in the building adjacent to Playboy X's Penthouse. This could be referring to the fact that since the characters of previous games are part of a separate canon, they will not appear or be referred to in later games as they were in the GTA III era. The characters may not be dead, but they exist outside of GTA IV's storyline.
There is a Sprunk manufacturing plant in Alderney. Sprunk was featured in GTA Vice City, GTA San Andreas, and Manhunt. It has returned to GTA IV with a brand new look. Also notice that "Sprunk" is a mix of the words "Sprite" and "Spunk". The sign outside the factory is very similar to a sign for Pepsi-Cola that appears in the film "The Great Gatsby."
One of the TV shows is a program called Las Venturas Poker. It features a casino from GTA San Andreas.
In Star Junction there is sign for Love Fist. Love Fist was featured in GTA Vice City, and was also on a sign in GTA San Andreas.
In the apartment in Bohan, next to the coffee table and in front of the bed, are a few CDs laying on the floor. Zoom in with a sniper to see that one of the CDs is by OG Loc from GTA San Andreas. There is also one by Madd Dogg.
There is a laundromat in Alderney called Mr. Wong's Laundromat; this is presumably either the same one from GTA III or a nod to that particular location.
When you kill Playboy X, go to his apartment and open the wardrobe to get a change of clothes that that look like the clothing of Claude, the protagonist of GTA III.
The Securicars are apparently still owned by the Gruppe Sechs security firm (as seen in GTA III and Vice City), although in GTA IV the name is rendered as "Gruppe 6".
In the bars are postcards featuring Vice City. Vice City is also advertised in airfare billboards near the airport.
The "Pager" ringtone on Niko's cell phone is music from Vice City.
Niko can receive an email from Chiliad8888, which calls the recipient "Pilgrim", makes many spiritually- and philosophically-charged (and anti-government) comments, ends with "Feel the Truth, Live the Freedom", and links to whattheydonotwantyoutoknow.com. On this website there is a discussion topic titled, "Jump to a new level of Spirituality" posted by "Trooth". This discussion thread includes more philosophical thoughts, mentions peyote, and provides a map of the Unique Stunt Jumps in Liberty City. "Chiliad" was also the name of a location in San Andreas (the highest mountain, in the southwestern wilderness). These clues suggest that the author of the email and discussion topic is The Truth, a character from GTA San Andreas.
On Happiness Island, in front of the entrance to the statue, it is possible to find a street-artist's easel/display stand with several pictures. These pictures are all concept works of many previous GTA characters. The easel can also be seen in south-central Algonquin, very close to where Hossan's random encounter takes place.
In Alderney, off Bowline and near the Mr. Fux's Rice Box eatery, is a restaurant named the "Greasy Joe's Cafe", a most likely reference to the Greasy Joe's back in GTA3. There is a little sideline mention in the restaurant section on Public Liberty Online dot com.
In Cluckin' Bell restaurants, near the back of the store, there's an action figure of a man in a martial arts position wearing a chicken hat; printed on the glass is "Cluck Norris," a reference to film/television star and martial artist Chuck Norris.
When the police get shot and don't die they sometimes scream "I going to be OK. I'm going to be OK!" A reference to the movie Reservoir Dogs.
The mission The Puerto Rican Connection is named after the movie The French Connection which features an El-Train chase, similiar to the mission.
Many street names in Bohan are named after prisons -- Leavenworth Ave, Alcatraz Ave, Sing Sing Ave, Guantanamo Ave, Attica Ave, San Quentin Ave, Folsom Way (best known from "Folsom Prison Blues" by Johnny Cash), Rykers Ave (named after the Rikers Island prison on the East River of New York City), Joliet Street.
Streets running east-to-west (horizontally) in Algonquin are named alphabetically after different types of minerals. "A" is at the southern end of Algonquin; "X" at the northern end. The names include Amethyst, Barium, Calcium, Diamond, Emerald, Feldspar, Garnet, Hematite, Iron, Jade, Kunzite, Lorimar, Manganese, Nickel, Obsidian, Pyrite, Quartz, Ruby, Silicon, Topaz, Uranium, Vauxite, Wardite, Xenotime
In the South of Alderney, the street names reflect (mostly antiquated) military terms -- Musketeer, Grenadier, Chariot, Phalanx etc.
In Broker, roads running North-South are named after Native American tribes -- Iroquois, Oneida, Mohawk, Onondaga etc.
In south Dukes and north Broker there are the street names "Cassidy" and "Sundance", a reference to the film Butch Cassidy and the Sundance Kid.
Broker contains the streets "Ringo St" and "Starr St", a reference to the Beatles member Ringo Starr.
Rockstar uses cartoon characters names as street names such as, "Bart (Simpson), (Hubert) Farnsworth" and the direct link to South Park, "South Parkway".
Some streets are named after tools -- Jackhammer, Plumbbob, Tinderbox.
Some streets are named after animals -- Caterpillar, Worm, Butterfly.
Some streets are named after foods -- Onion, Asparagus, Chive.
Some streets are named after colonial Settlements -- Yorktown, Delaware.
Some streets are named after famous surnames, -- Harrison, Hancock, Livingstone.
Some streets are named after nuclear tests -- Plumbbob, Emery, and Niblick.
On the Algonquin Bridge from Algonquin to Broker, on the right is a building with the name "CLOCKTOWER"; this is the reminiscent of the "WATCHTOWER" building of the Society of the Jehovah's Witnesses in Brooklyn.
On the stairway leading to the sea at Meadow Hills there is a 'Banksy-esque' style graffiti piece.
On the sides of some Sentinels are a little pink "STD," referring to Subaru's pink "STi".
There is a Liberty Tree article that talks about urban myths in Liberty City. This could be referring to the myths that were created around San Andreas.
An online article on Liberty Tree's website, an Air Sol flight has 212 passengers onboard, 212 a reference to the area code in New York City.
In Willis, there is a shop called Al's Furniture. This is an obvious reference to Italian-American gangster Al Capone, who was officially the owner of a furniture shop.
The outside sign for the Memory Lanes bowling alley depicts a bowling pin with two bowling balls on either side of it. Inside, however, the sign is only a neon outline, and resembles a crude drawing of male sex organs.
In many spots around Liberty City, there is rubble on the ground; in some of this rubble, you can see a purple dildo sticking out.
There are signs scattered throughout Liberty City that say 'Hot Coffee Shop,' clearly a reference to the Hot Coffee sexual content and controversy from GTA San Andreas
Looking at the golf courses carefully from a height reveals that they are penis-shaped.
A brand of beer in the game is called 'Dusche Gold.' "Dusche" is German for "shower", so it translates into "Shower of Gold."
Another beer brand (presumably spoofing Budweiser) is named "Piswasser", "wasser" being German for "water".
The logo for AutoErotica is a car resembling a penis