I know you can ride a cab in GTA IV. How do you hail a cab in GTA IV? I own the PS3 version.
You can start hailing cabs when the game gives you instructions on how to do it. This is after the first Vlad mission.
When a cab is close by press the L1 button and Niko will whistle to hail the cab. When the cab stops for you hold the triangle button until Niko gets in the cab. It is very important that you hold the triangle button because if you tap it Niko will steal (car-jack) the cab instead.
To hail a cab in GTA IV 360 version press the LB button to whistle when a cab is close by and hold the Y button to get in the cab.