渎 originally meant 'a river that flows directly to the sea' - a specific geographic category, not just any waterway. The Ji (济水) has since silted up and no longer reaches the sea, so the classical 'four' can feel like three to a modern reader.
culture
The 四渎 were imperial sacrificial rivers - emperors offered ceremonies to them as part of state ritual, alongside the 五岳 (Five Sacred Mountains).
