You cannot extract this file by itself. You must have all preceding parts ( part1 , part2 , part3 ) and any subsequent parts in the same folder.
Watch Dogs 2 - Title Update 1.16 (4 Player Co-op, Fireworks, & More!)
Ensure you aren't missing any .rar files in the sequence.
If the game won't launch, try running it in Windows 7/8 Compatibility Mode or as an Administrator via the application properties.