When driving at night, a following distance of 4 seconds allows your low beam lights to illuminate:

When driving at night, your low beam headlights typically have a range that is considerably limited compared to driving during the day. A following distance of 4 seconds is generally recommended for maintaining safe spacing between vehicles in clear weather, but at night, visibility becomes a critical factor.

With a following distance of 4 seconds, you're allowing for appropriate reaction time, but the low beam headlights can only illuminate a limited area ahead. This is approximately about half the distance to the vehicle in front of you. Given that low beams cast light at a shorter range, it would not extend the entire 4 seconds of travel distance (which could be much longer on the highway or at speed). Thus, your headlights effectively cover a distance that is roughly half of the total following distance measured in time, necessitating greater caution as the visibility is compromised during nighttime driving.