After the Three Leaf Clover mission is completed, Ivan is unlocked as a random character and can be approached for a side mission.

To gain access to this mission, Ivan's life had to be spared in the earlier mission Ivan the Not So Terrible (acquired from Vlad). Find Ivan in Acter, Alderney, notated by the blue "friend" icon on the map. Ivan is a loan shark and needs protection during a loan shark deal. The mission soon turns into a gun fight, but if you succeed in protecting Ivan and eliminating the hostiles, Ivan will give you a generous reward.

As noted above, the player has the option to either kill or save Ivan during the Ivan the Not So Terrible mission. There are a number of instances similar to this in the game, which have varying consequences on how the game plays out. Some of the characters will merely continue to exist, some may offer the player additional missions, and some may lead to unlocking new items, such as safe houses.IGN: Grand Theft Auto IV Game Guide