Class 4 is one of the trickier classes to learn because its three divisions look and behave very differently. They are all “flammable solids” in name, but the way each one becomes dangerous is distinct, and the placards reflect that with three different looks.
This is study guidance, not regulatory advice. The binding definitions are in 49 CFR 173.124 and your official state CDL manual.
The three divisions
| Division | Name | Placard look | Examples |
|---|---|---|---|
| 4.1 | Flammable solid | Red-and-white vertical stripes | Matches, sulfur, nitrocellulose |
| 4.2 | Spontaneously combustible | White top half, red bottom half | White phosphorus, some metal powders |
| 4.3 | Dangerous when wet | Solid blue | Sodium, potassium, calcium carbide |
Each division ignites a different way: 4.1 burns readily once lit, 4.2 can catch fire on its own without an outside spark, and 4.3 reacts with water to release flammable gas.
Division 4.1: flammable solids
Division 4.1 covers solids that ignite easily through friction, a spark, or a small flame. The placard is the one with red and white vertical stripes, a flame symbol, and a 4 at the bottom. Matches are the textbook example.
Division 4.2: spontaneously combustible
Division 4.2 materials can heat up and ignite on their own, with no external ignition source, often through contact with air. The placard is white on the top half and red on the bottom, which is a common point of confusion with the striped 4.1. Reading the placard correctly, stripe pattern and all, is what separates them, and the pair shows up in most confused hazmat placards.
Division 4.3: dangerous when wet
Division 4.3 is the blue placard, the only blue diamond in the system, and it gets its own deep dive in the dangerous-when-wet placard explained. The key fact for Class 4: 4.3 is a Table 1 material, placarded in any amount, while 4.1 and 4.2 are Table 2 materials under the 1,001-pound rule.
Where it fits
Class 4 is one of the nine hazard classes, and its split personality makes it a favorite test subject. For the regulations and response, see the FMCSA hazardous materials regulations, the placarding rule in 49 CFR 172.504, and the PHMSA Emergency Response Guidebook.
Frequently asked questions
What are the three Class 4 divisions?
They are 4.1 flammable solids (red-and-white stripes), 4.2 spontaneously combustible (white top, red bottom), and 4.3 dangerous when wet (blue). Each becomes dangerous in a different way.
How do I tell division 4.1 from 4.2?
Look at the placard pattern. Division 4.1 has red and white vertical stripes, while 4.2 is white on the top half and red on the bottom half. Both show a flame and the number 4.
Which Class 4 division must always be placarded?
Division 4.3, dangerous when wet, is a Table 1 material and must be placarded in any amount. Divisions 4.1 and 4.2 are Table 2 materials, placarded at 1,001 pounds or more.
What is the best way to study the Class 4 placards?
Learn the three looks and what each division does, then drill them against each other with a recognition app such as CDL Placards. Your state CDL manual is the authority on the definitions and rules.
