If a vehicle carrying a Category III-YELLOW industrial radiography source (e.g. Ir-192) suffers a breakdown on a remote highway, what security duty rests on the driver under ADR 8.4?

1. Leave the keys in the ignition and walk to a diner 5 miles away
2. Hitchhike away and return two days later
3. Give the keys to a stranger passing by
4. Maintain continuous personal supervision or secure the vehicle in an approved guarded compound; never leave the radioactive load unattended