Vault 4 is hidden in the Wasteland; you'll need to search around to find it.
The lone security robot has fairly serious anger issues, and will attack anyone that moves into its sight. Since the only exit once you're at the apartments tends to lead straight past the robot, you'd be well advised to steer clear until you're confident in your abilities to run like a sissy girl.
Rent: $10000 down payment and $100 a month
Security: Crazy fuckin' robot, general obscurity.