To do this you need to replace your save file which can be accessed through - "C:\Documents\Rockstar Games\GTA IV\Profiles"

The save file can be downloaded here > https://drive.google.com/file/d/1gU8L1Pmp3dbczIKhnsRiv4dSjzK52MXC/view

Don't forget to back up the save file it replaces.

You can also use this cheat phone number to spawn a helicopter - 359-555-0100 (Blocks 'One Man Army' achievement)