Stunt Jumps in GTA IV
There are 50 stunt jumps in the game.
If you are successful, the game will report "Stunt jump completed" then "NN stump jumps left to do."
List of all stunt jumps
At the top of Baobo, get on the unused tracks, heading West. There is a wooden ramp at the end. You can take it at speed in most vehicles to land on the promenade below (or crash into the water) to get the jump.
From an alley on the border of Baobao and Steinway, heading West. There is a wooden ramp at the end of the alley. You need to land fairly far in the junkyard below. A Turismo can do it.