The Ballad of Gay Tony: Niko and Johnny easter eggs
When the game begins, when Luis is shown walking down the streets of Liberty City, you can see Niko in a vehicle after the heist. You will also see Johnny ride off on his motorcycle.
When the game begins, during the heist, Niko and the Irish family of Patrick McReary are the robbers. Unfortunately, Michael gets shot, just as in the storyline in Grand Theft Auto 4.
During the introduction sequence of Grand Theft Auto 4, you can see a chef placing diamonds into a cake. What the man is trying to do is import millions of dollars worth of diamonds to sell them in Liberty City. The buyer happens to be Tony, but he does not seem to be the only one who wants them. From a different point of view, you can see that the Lost gang is trying to come up behind their back, along with Johnny.
When Tony tells you to retrieve the diamonds back from the Lost gang before they sell them, Luis has to go to the Libertonian museum and ambush them. After he shoots the man inspecting the diamonds, notice that it is Niko and Johnny trying to sell them.