I think Rockstar probably had a least the plot outlines written for DLC when they scripted GTA4. Take the diamond missions for example. Johnny runs off with the money, and it's never really explained what his motive is, there's only a line in a follow-up mission when a biker sees Niko and says "There's the guy Johnny ripped for the diamond loot." I suppose you could take it to mean that he just saw an opportunity for a big payoff but, the appearance of Luis in the second floor window. Now where have I seen that Letterman jacket before...? Wait, wasn't he at the bank that day...?
But I digress. The point is, they probably would have released something with or without MS's money. First-day sales of GTA4 amounted to $310 million in revenue for Rockstar. Granted it probably wasn't all profit but they definiteky had the funds to make DLC. GTA4 had a production cost of just over $100 million. DLC surely doesn't reach that, does it?