Which is a characteristic of Class 4: Flammable Solids?

Class 4: Flammable Solids is categorized specifically by their ability to easily catch fire. This classification covers materials that can ignite through friction, moisture absorption or spontaneous chemical changes. The designation of "flammable" implies that these solids have a high likelihood of igniting under normal conditions, such as during handling or transportation.

In contrast, the other options present characteristics that do not align with Class 4. The statement regarding materials that do not ignite easily is contrary to the definition of flammable solids, which are inherently at risk of ignition. The idea that they are always non-combustible directly contradicts their classification, as flammable solids are specifically known for their combustibility. Lastly, while some flammable solids may have a low flash point, this is not a defining characteristic of the class, which is more focused on their propensity to catch fire easily rather than specific temperature metrics. Thus, the correct answer is anchored in the defining nature of Class 4 materials as readily ignitable.