What distinguishes Class 4.1 (flammable solids) from Class 4.2 (substances liable to spontaneous combustion) under ADR classification?

1. Class 4.1 requires an external ignition source to burn, whereas Class 4.2 can ignite without external energy input
2. Class 4.1 contains liquids that emit flammable gases, whereas Class 4.2 only contains solids
3. Class 4.1 covers substances that react violently with water, whereas Class 4.2 covers substances that are stable in water
4. Class 4.1 substances are toxic, whereas Class 4.2 substances are exclusively corrosive