So I was just reading Tavell Clinton (Franklin's cousin)'s LifeInvader page, and he has a couple of posts that reference GTA IV. He lives in Liberty City, and some of his posts mention stuff from IV. For example, on of his posts are "Roman's Taxi Service - worst cab company ever!".
I thought that was the only reference, but then I saw one of his first posts saying that he's renting an apartment with "two Jamaican dudes in Schottler". Little Jacob and Real Badman live together in GTA IV, their apartment is in Schottler. I'm positive that this is a reference to Jacob, especially since he says "now there's 3 black men living together who don't understand each other".