What device should be installed at the front of a moving work convoy (such as a slow-moving lane striping operation) on a multi-lane highway?

1. A small walking flagger carrying a handheld STOP paddle.
2. A stationary water tank trailer parked along the shoulder of the road only.
3. A standard passenger automobile with no flashing lights.
4. An Advance Warning Shadow Vehicle equipped with an Arrow Board and TMA.