Which of the following is true regarding 'Stop Bars' installed at taxiway holding positions?

1. They consist of a row of red lights showing across the taxiway; aircraft must never cross a lit stop bar, even if a voice clearance is received.
2. They consist of flashing yellow lights indicating caution; pilots may cross them at their own discretion.
3. They are only switched on during daylight hours to guide construction vehicles.
4. Crossing a lit stop bar is permitted if the aircraft is operating under Visual Flight Rules (VFR).