What is the purpose of the 'Emergency Roof Escape Hatch' (Notausstiegsluke im Dach) on a passenger coach?

1. To allow passengers to stargaze at night
2. To ventilate smoke from the driver's cigarette
3. To load luggage through the roof
4. To provide an emergency egress portal for passengers if the coach rolls over onto its side and both side doors and windows are obstructed against the ground or guardrails