Back to IV, I just love the dynamic between Niko, Roman, Florian and Darko.
All four of them grew up in horrible conditions in Eastern Europe. Niko, Florian and Darko fought in an unspecified war, which has affected all of them in different ways.
Niko wanted answers, Darko turned into an addict, Florian transformed into somebody else. I feel Niko is stuck in the middle between the two. Florian is a foil to Niko's quest for revenge by moving on into a new city, while Darko could be considered a warning to Niko's future if he doesn't move on.
Probably for the best that Niko is self-contained in IV, who knows how Niko is coping from either ending.