Brucie Kibbutz from Grand Theft Auto 4 returns to Grand Theft Auto 5 on Bleeper (GTA’s equivalent to Twitter). Niko Bellic's old friend Packie McReary also returns to GTA 5. Franklin can help him during a heist, and he will join your crew for the remainder of the game.
One of Franklin's friends on his LifeInvader page is "Nicholas Bell". This is a reference to "Niko Bellic", GTA 4's protagonist.
While at Michael's home, zoom into Jimmy when he is using his laptop at the dinner table to see him viewing the LifeInvader profile for Niko Bellic.
One of the movies you can watch at the cinema shows the beating heart inside the Happiness Island statue that appeared in GTA IV.
In the first mission for setting up the jewelry heist, when Lester is telling Michael that all the old crew is gone, he mentions a eastern European guy from Liberty City.
At the end of the "Monkey Business" mission, the guy in the suit with glasses who works for the lab that you are stealing from tells the guards to detain the FIB agent, who shoots himself in the leg to cover for his friends, is the same person known as Edward T. Fortune that gives Niko the information he seeks in return for him doing missions for the United Liberty Paper in Grand Theft Auto 4. He also appears later in "The Wrap Up" mission. He shows up during the stand off between De Santa, Haines, Norton, and Agent Sanchez, alongside several IAA agents.
In "The Paleto Score" mission, the four characters (Franklin, Michael, Trevor, and whomever you chose as a crew member) will talk about there first big score. If you chose Patrick "Packie" McReary, he will talk about his first big score being the Bank of Liberty City. He will list two names, and after each will say "God rest his soul", then he says something like "...and my friend Niko who is probably dead."
Johnny Klebitz from The Lost And Damned DLC accuses Trevor of sleeping with his girlfriend, Ashley.