Sodium reacts dangerously with water and is listed in the CDL Manual as an example material for which hazard class division?

AAMVA CDL Manual, Section 9.3.1, Figure 9.1 (nhcdm.pdf p. 9-2); 49 CFR 173.124

A Dangerous-When-Wet load reacts violently in rain, a washed-out road, or a firefighter's hose stream, so misidentifying the division can turn a routine spill response into an explosion or toxic-gas release. Drivers hauling Class 4 materials need to recognize this specific division on sight, not just know "Class 4 is flammable."