Here's why:

* Isotopes: We now know that atoms of the same element can have different numbers of neutrons. These different forms are called isotopes. While isotopes of the same element share the same number of protons (and thus the same atomic number), they have different masses due to the varying number of neutrons. This contradicts Dalton's idea of identical mass for all atoms of an element.

* Nuclear Reactions: Dalton's theory predates the discovery of nuclear reactions. We now understand that atoms can be split, fused, and even transformed into other elements through nuclear processes. This demonstrates that atoms are not indivisible as Dalton believed.

However, Dalton's fourth postulate can be considered a valid approximation for many chemical reactions. In most everyday chemical reactions, the differences in mass and properties between isotopes are insignificant. The chemical behavior of atoms is primarily determined by their electron configuration, which is largely determined by the number of protons.

Therefore, while Dalton's fourth postulate is not completely accurate, it provided a foundational framework for the development of modern atomic theory. It was a significant step forward in our understanding of matter, even though it has been refined and expanded upon.