After the cutscene ends, you will have a Blista Compact in front of you, which was stolen in Clean Getaway. You can use it or your own vehicle (by stealing or parking near the safehouse), but the Blista Compact is fast enough that it’s recommended for the chase.
After choosing a vehicle, drive to Roman’s Car Depot. There you will see Ivan trying to escape in a Ruiner. Start chasing him immediately. Be careful, this is the hardest part of the mission as there are some scripted vehicles like trucks and vans which can be easily bumped into and fail the mission. The trick is to be slow and accelerate steadily. Even though it seems like Ivan is accelerating a lot, he slows down every now and then, and sometimes due to the fault in AI, he himself bumps into some vehicles.
After chasing him to the construction site, he will crash his vehicle and start climbing the ladder. To climb a ladder, press F on PC, Y on Xbox, or Triangle on PlayStation to hold the ladder and then W on PC, or move the left analog up for PlayStation and Xbox. To climb quickly, press and hold SHIFT on PC, repeatedly press A on Xbox, or Cross on PlayStation.
There are 3 ladders needed to climb in this mission. Also, be careful when jumping over buildings, as if you fall, you will get wasted, and the mission will fail. After you chase him till the end of the buildings, he will slip down and ask for your help to save him. This is when you have to make the first choice in the game.
Choices in GTA 4
GTA 4 offers choices in some missions which affect the story in some ways. Even though the choices don’t affect the storyline that much as they do in role-playing games, they can bring you some benefits or disadvantages. Although there are usually no right or wrong choices, unlike the one before the last mission in GTA 5, be careful when making them.
Option #1 — Kill Ivan
If you press Q on PC, X on Xbox, or Square on PlayStation, Niko will kick Ivan off the edge and kill him. It won’t affect the story in any way.
Option #2 — Save Ivan
If you press E on PC, LB on Xbox, or L1 on PlayStation, Niko will lift Ivan up and save him. It unlocks a random encounter at a later part of the game on the third island.
There is a glitch with Ivan’s car. If the player destroys the vehicle, thus killing Ivan, the mission will still be to “chase Ivan”. The only way to get out of this is by failing the mission, leaving the area, or dying.
The whole traffic during the chase is scripted.
It is possible to kill Ivan before he reaches the ledge if the player shoots him enough while he is running away.
This is the first mission in GTA 4 as well as the whole GTA franchise to offer a choice to the player.